diff --git a/core/sysprop.mk b/core/sysprop.mk
index 0cee81c..3806a96 100644
--- a/core/sysprop.mk
+++ b/core/sysprop.mk
@@ -71,10 +71,11 @@
 define build-properties
 ALL_DEFAULT_INSTALLED_MODULES += $(2)
 
-# TODO(b/117892318): eliminate the call to uniq-pairs-by-first-component when
-# it is guaranteed that there is no dup.
+$(eval # Properties can be assigned using `prop ?= value` or `prop = value` syntax.)
+$(eval # Eliminate spaces around the ?= and = separators.)
 $(foreach name,$(strip $(4)),\
-    $(eval _resolved_$(name) := $$(call collapse-pairs, $$(call uniq-pairs-by-first-component,$$($(name)),=)))\
+    $(eval _temp := $$(call collapse-pairs,$$($(name)),?=))\
+    $(eval _resolved_$(name) := $$(call collapse-pairs,$$(_temp),=))\
 )
 
 $(2): $(POST_PROCESS_PROPS) $(INTERNAL_BUILD_ID_MAKEFILE) $(API_FINGERPRINT) $(3)
diff --git a/tools/Android.bp b/tools/Android.bp
index 159890c..149d06d 100644
--- a/tools/Android.bp
+++ b/tools/Android.bp
@@ -37,3 +37,22 @@
     },
   },
 }
+
+python_test_host {
+  name: "post_process_props_unittest",
+  main: "test_post_process_props.py",
+  srcs: [
+    "post_process_props.py",
+    "test_post_process_props.py",
+  ],
+  version: {
+    py2: {
+      enabled: false,
+    },
+    py3: {
+      enabled: true,
+    },
+  },
+  test_config: "post_process_props_unittest.xml",
+  test_suites: ["general-tests"],
+}
diff --git a/tools/post_process_props.py b/tools/post_process_props.py
index 4fa15bc..397526f 100755
--- a/tools/post_process_props.py
+++ b/tools/post_process_props.py
@@ -16,8 +16,8 @@
 
 import sys
 
-# Usage: post_process_props.py file.prop [blacklist_key, ...]
-# Blacklisted keys are removed from the property file, if present
+# Usage: post_process_props.py file.prop [disallowed_key, ...]
+# Disallowed keys are removed from the property file, if present
 
 # See PROP_VALUE_MAX in system_properties.h.
 # The constant in system_properties.h includes the terminating NUL,
@@ -29,8 +29,8 @@
 def mangle_build_prop(prop_list):
   # If ro.debuggable is 1, then enable adb on USB by default
   # (this is for userdebug builds)
-  if prop_list.get("ro.debuggable") == "1":
-    val = prop_list.get("persist.sys.usb.config")
+  if prop_list.get_value("ro.debuggable") == "1":
+    val = prop_list.get_value("persist.sys.usb.config")
     if "adb" not in val:
       if val == "":
         val = "adb"
@@ -40,52 +40,121 @@
   # UsbDeviceManager expects a value here.  If it doesn't get it, it will
   # default to "adb". That might not the right policy there, but it's better
   # to be explicit.
-  if not prop_list.get("persist.sys.usb.config"):
+  if not prop_list.get_value("persist.sys.usb.config"):
     prop_list.put("persist.sys.usb.config", "none");
 
 def validate(prop_list):
   """Validate the properties.
 
+  If the value of a sysprop exceeds the max limit (91), it's an error, unless
+  the sysprop is a read-only one.
+
+  Checks if there is no optional prop assignments.
+
   Returns:
     True if nothing is wrong.
   """
   check_pass = True
-  for p in prop_list.get_all():
+  for p in prop_list.get_all_props():
     if len(p.value) > PROP_VALUE_MAX and not p.name.startswith("ro."):
       check_pass = False
       sys.stderr.write("error: %s cannot exceed %d bytes: " %
                        (p.name, PROP_VALUE_MAX))
       sys.stderr.write("%s (%d)\n" % (p.value, len(p.value)))
+
+    if p.is_optional():
+      check_pass = False
+      sys.stderr.write("error: found unresolved optional prop assignment:\n")
+      sys.stderr.write(str(p) + "\n")
+
   return check_pass
 
+def override_optional_props(prop_list):
+  """Override a?=b with a=c, if the latter exists
+
+  Overriding is done by deleting a?=b
+  When there are a?=b and a?=c, then only the last one survives
+  When there are a=b and a=c, then it's an error.
+
+  Returns:
+    True if the override was successful
+  """
+  success = True
+  for name in prop_list.get_all_names():
+    props = prop_list.get_props(name)
+    optional_props = [p for p in props if p.is_optional()]
+    overriding_props = [p for p in props if not p.is_optional()]
+    if len(overriding_props) > 1:
+      # duplicated props are allowed when the all have the same value
+      if all(overriding_props[0].value == p.value for p in overriding_props):
+        continue
+      success = False
+      sys.stderr.write("error: found duplicate sysprop assignments:\n")
+      for p in overriding_props:
+        sys.stderr.write("%s\n" % str(p))
+    elif len(overriding_props) == 1:
+      for p in optional_props:
+        p.delete("overridden by %s" % str(overriding_props[0]))
+    else:
+      if len(optional_props) > 1:
+        for p in optional_props[:-1]:
+          p.delete("overridden by %s" % str(optional_props[-1]))
+      # Make the last optional one as non-optional
+      optional_props[-1].optional = False
+
+  return success
+
 class Prop:
 
-  def __init__(self, name, value, comment=None):
+  def __init__(self, name, value, optional=False, comment=None):
     self.name = name.strip()
     self.value = value.strip()
-    self.comment = comment
+    if comment != None:
+      self.comments = [comment]
+    else:
+      self.comments = []
+    self.optional = optional
 
   @staticmethod
   def from_line(line):
     line = line.rstrip('\n')
     if line.startswith("#"):
-      return Prop("", "", line)
+      return Prop("", "", comment=line)
+    elif "?=" in line:
+      name, value = line.split("?=", 1)
+      return Prop(name, value, optional=True)
     elif "=" in line:
       name, value = line.split("=", 1)
-      return Prop(name, value)
+      return Prop(name, value, optional=False)
     else:
       # don't fail on invalid line
       # TODO(jiyong) make this a hard error
-      return Prop("", "", line)
+      return Prop("", "", comment=line)
 
   def is_comment(self):
-    return self.comment != None
+    return bool(self.comments and not self.name)
+
+  def is_optional(self):
+    return (not self.is_comment()) and self.optional
+
+  def make_as_comment(self):
+    # Prepend "#" to the last line which is the prop assignment
+    if not self.is_comment():
+      assignment = str(self).rsplit("\n", 1)[-1]
+      self.comments.append("#" + assignment)
+      self.name = ""
+      self.value = ""
+
+  def delete(self, reason):
+    self.comments.append("# Removed by post_process_props.py because " + reason)
+    self.make_as_comment()
 
   def __str__(self):
-    if self.is_comment():
-      return self.comment
-    else:
-      return self.name + "=" + self.value
+    assignment = []
+    if not self.is_comment():
+      operator = "?=" if self.is_optional() else "="
+      assignment.append(self.name + operator + self.value)
+    return "\n".join(self.comments + assignment)
 
 class PropList:
 
@@ -94,25 +163,35 @@
       self.props = [Prop.from_line(l)
                     for l in f.readlines() if l.strip() != ""]
 
-  def get_all(self):
+  def get_all_props(self):
     return [p for p in self.props if not p.is_comment()]
 
-  def get(self, name):
+  def get_all_names(self):
+    return set([p.name for p in self.get_all_props()])
+
+  def get_props(self, name):
+    return [p for p in self.get_all_props() if p.name == name]
+
+  def get_value(self, name):
+    # Caution: only the value of the first sysprop having the name is returned.
     return next((p.value for p in self.props if p.name == name), "")
 
   def put(self, name, value):
-    index = next((i for i,p in enumerate(self.props) if p.name == name), -1)
+    # Note: when there is an optional prop for the name, its value isn't changed.
+    # Instead a new non-optional prop is appended, which will override the
+    # optional prop. Otherwise, the new value might be overridden by an existing
+    # non-optional prop of the same name.
+    index = next((i for i,p in enumerate(self.props)
+                  if p.name == name and not p.is_optional()), -1)
     if index == -1:
-      self.props.append(Prop(name, value))
+      self.props.append(Prop(name, value,
+                             comment="# Auto-added by post_process_props.py"))
     else:
+      self.props[index].comments.append(
+          "# Value overridden by post_process_props.py. Original value: %s" %
+          self.props[index].value)
       self.props[index].value = value
 
-  def delete(self, name):
-    index = next((i for i,p in enumerate(self.props) if p.name == name), -1)
-    if index != -1:
-      new_comment = "# removed by post_process_props.py\n#" + str(self.props[index])
-      self.props[index] = Prop.from_line(new_comment)
-
   def write(self, filename):
     with open(filename, 'w+') as f:
       for p in self.props:
@@ -127,12 +206,15 @@
 
   props = PropList(filename)
   mangle_build_prop(props)
+  if not override_optional_props(props):
+    sys.exit(1)
   if not validate(props):
     sys.exit(1)
 
-  # Drop any blacklisted keys
+  # Drop any disallowed keys
   for key in argv[2:]:
-    props.delete(key)
+    for p in props.get_props(key):
+      p.delete("%s is a disallowed key" % key)
 
   props.write(filename)
 
diff --git a/tools/post_process_props_unittest.xml b/tools/post_process_props_unittest.xml
new file mode 100644
index 0000000..4a6ecc2
--- /dev/null
+++ b/tools/post_process_props_unittest.xml
@@ -0,0 +1,6 @@
+<configuration description="Config to run post_process_props_unittest">
+    <test class="com.android.tradefed.testtype.python.PythonBinaryHostTest" >
+        <option name="par-file-name" value="post_process_props_unittest" />
+        <option name="test-timeout" value="1m" />
+    </test>
+</configuration>
diff --git a/tools/test_post_process_props.py b/tools/test_post_process_props.py
new file mode 100644
index 0000000..8a9c3ed
--- /dev/null
+++ b/tools/test_post_process_props.py
@@ -0,0 +1,230 @@
+#!/usr/bin/env python3
+#
+# Copyright (C) 2020 The Android Open Source Project
+#
+# Licensed under the Apache License, Version 2.0 (the "License");
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+#
+#      http://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
+
+import contextlib
+import io
+import unittest
+
+from unittest.mock import *
+from post_process_props import *
+
+class PropTestCase(unittest.TestCase):
+  def test_createFromLine(self):
+    p = Prop.from_line("# this is comment")
+    self.assertTrue(p.is_comment())
+    self.assertEqual("", p.name)
+    self.assertEqual("", p.value)
+    self.assertFalse(p.is_optional())
+    self.assertEqual("# this is comment", str(p))
+
+    for line in ["a=b", "a = b", "a= b", "a =b", "  a=b   "]:
+      p = Prop.from_line(line)
+      self.assertFalse(p.is_comment())
+      self.assertEqual("a", p.name)
+      self.assertEqual("b", p.value)
+      self.assertFalse(p.is_optional())
+      self.assertEqual("a=b", str(p))
+
+    for line in ["a?=b", "a ?= b", "a?= b", "a ?=b", "  a?=b   "]:
+      p = Prop.from_line(line)
+      self.assertFalse(p.is_comment())
+      self.assertEqual("a", p.name)
+      self.assertEqual("b", p.value)
+      self.assertTrue(p.is_optional())
+      self.assertEqual("a?=b", str(p))
+
+  def test_makeAsComment(self):
+    p = Prop.from_line("a=b")
+    p.comments.append("# a comment")
+    self.assertFalse(p.is_comment())
+
+    p.make_as_comment()
+    self.assertTrue(p.is_comment())
+    self.assertTrue("# a comment\n#a=b", str(p))
+
+class PropListTestcase(unittest.TestCase):
+  def setUp(self):
+    content = """
+    # comment
+    foo=true
+    bar=false
+    qux?=1
+    # another comment
+    foo?=false
+    """
+    self.patcher = patch("post_process_props.open", mock_open(read_data=content))
+    self.mock_open = self.patcher.start()
+    self.props = PropList("file")
+
+  def tearDown(self):
+    self.patcher.stop()
+    self.props = None
+
+  def test_readFromFile(self):
+    self.assertEqual(4, len(self.props.get_all_props()))
+    expected = [
+        ("foo", "true", False),
+        ("bar", "false", False),
+        ("qux", "1", True),
+        ("foo", "false", True)
+    ]
+    for i,p in enumerate(self.props.get_all_props()):
+      self.assertEqual(expected[i][0], p.name)
+      self.assertEqual(expected[i][1], p.value)
+      self.assertEqual(expected[i][2], p.is_optional())
+      self.assertFalse(p.is_comment())
+
+    self.assertEqual(set(["foo", "bar", "qux"]), self.props.get_all_names())
+
+    self.assertEqual("true", self.props.get_value("foo"))
+    self.assertEqual("false", self.props.get_value("bar"))
+    self.assertEqual("1", self.props.get_value("qux"))
+
+    # there are two assignments for 'foo'
+    self.assertEqual(2, len(self.props.get_props("foo")))
+
+  def test_putNewProp(self):
+    self.props.put("new", "30")
+
+    self.assertEqual(5, len(self.props.get_all_props()))
+    last_prop = self.props.get_all_props()[-1]
+    self.assertEqual("new", last_prop.name)
+    self.assertEqual("30", last_prop.value)
+    self.assertFalse(last_prop.is_optional())
+
+  def test_putExistingNonOptionalProp(self):
+    self.props.put("foo", "NewValue")
+
+    self.assertEqual(4, len(self.props.get_all_props()))
+    foo_prop = self.props.get_props("foo")[0]
+    self.assertEqual("foo", foo_prop.name)
+    self.assertEqual("NewValue", foo_prop.value)
+    self.assertFalse(foo_prop.is_optional())
+    self.assertEqual("# Value overridden by post_process_props.py. " +
+                     "Original value: true\nfoo=NewValue", str(foo_prop))
+
+  def test_putExistingOptionalProp(self):
+    self.props.put("qux", "2")
+
+    self.assertEqual(5, len(self.props.get_all_props()))
+    last_prop = self.props.get_all_props()[-1]
+    self.assertEqual("qux", last_prop.name)
+    self.assertEqual("2", last_prop.value)
+    self.assertFalse(last_prop.is_optional())
+    self.assertEqual("# Auto-added by post_process_props.py\nqux=2",
+                     str(last_prop))
+
+  def test_deleteNonOptionalProp(self):
+    props_to_delete = self.props.get_props("foo")[0]
+    props_to_delete.delete(reason="testing")
+
+    self.assertEqual(3, len(self.props.get_all_props()))
+    self.assertEqual("# Removed by post_process_props.py because testing\n" +
+                     "#foo=true", str(props_to_delete))
+
+  def test_deleteOptionalProp(self):
+    props_to_delete = self.props.get_props("qux")[0]
+    props_to_delete.delete(reason="testing")
+
+    self.assertEqual(3, len(self.props.get_all_props()))
+    self.assertEqual("# Removed by post_process_props.py because testing\n" +
+                     "#qux?=1", str(props_to_delete))
+
+  def test_overridingNonOptional(self):
+    props_to_be_overridden = self.props.get_props("foo")[1]
+    self.assertTrue("true", props_to_be_overridden.value)
+
+    self.assertTrue(override_optional_props(self.props))
+
+    # size reduced to 3 because foo?=false was overridden by foo=true
+    self.assertEqual(3, len(self.props.get_all_props()))
+
+    self.assertEqual(1, len(self.props.get_props("foo")))
+    self.assertEqual("true", self.props.get_props("foo")[0].value)
+
+    self.assertEqual("# Removed by post_process_props.py because " +
+                     "overridden by foo=true\n#foo?=false",
+                     str(props_to_be_overridden))
+
+  def test_overridingOptional(self):
+    content = """
+    # comment
+    qux?=2
+    foo=true
+    bar=false
+    qux?=1
+    # another comment
+    foo?=false
+    """
+    with patch('post_process_props.open', mock_open(read_data=content)) as m:
+      props = PropList("hello")
+
+      props_to_be_overridden = props.get_props("qux")[0]
+      self.assertEqual("2", props_to_be_overridden.value)
+
+      self.assertTrue(override_optional_props(props))
+
+      self.assertEqual(1, len(props.get_props("qux")))
+      self.assertEqual("1", props.get_props("qux")[0].value)
+      # the only left optional assignment becomes non-optional
+      self.assertFalse(props.get_props("qux")[0].is_optional())
+
+      self.assertEqual("# Removed by post_process_props.py because " +
+                       "overridden by qux?=1\n#qux?=2",
+                       str(props_to_be_overridden))
+
+  def test_overridingDuplicated(self):
+    content = """
+    # comment
+    foo=true
+    bar=false
+    qux?=1
+    foo=false
+    # another comment
+    foo?=false
+    """
+    with patch("post_process_props.open", mock_open(read_data=content)) as m:
+      stderr_redirect = io.StringIO()
+      with contextlib.redirect_stderr(stderr_redirect):
+        props = PropList("hello")
+
+        # fails due to duplicated foo=true and foo=false
+        self.assertFalse(override_optional_props(props))
+
+        self.assertEqual("error: found duplicate sysprop assignments:\n" +
+                         "foo=true\nfoo=false\n", stderr_redirect.getvalue())
+
+  def test_overridingDuplicatedWithSameValue(self):
+    content = """
+    # comment
+    foo=true
+    bar=false
+    qux?=1
+    foo=true
+    # another comment
+    foo?=false
+    """
+    with patch("post_process_props.open", mock_open(read_data=content)) as m:
+      stderr_redirect = io.StringIO()
+      with contextlib.redirect_stderr(stderr_redirect):
+        props = PropList("hello")
+
+        # we have duplicated foo=true and foo=true, but that's allowed
+        # since they have the same value
+        self.assertTrue(override_optional_props(props))
+
+if __name__ == '__main__':
+    unittest.main(verbosity=2)