diff --git a/core/board_config.mk b/core/board_config.mk
index ae1614f..1fbd989 100644
--- a/core/board_config.mk
+++ b/core/board_config.mk
@@ -93,6 +93,7 @@
   BUILD_BROKEN_TREBLE_SYSPROP_NEVERALLOW \
   BUILD_BROKEN_USES_NETWORK \
   BUILD_BROKEN_VINTF_PRODUCT_COPY_FILES \
+  BUILD_BROKEN_DUP_SYSPROP \
 
 _build_broken_var_list += \
   $(foreach m,$(AVAILABLE_BUILD_MODULE_TYPES) \
diff --git a/core/sysprop.mk b/core/sysprop.mk
index 3806a96..aa4fe91 100644
--- a/core/sysprop.mk
+++ b/core/sysprop.mk
@@ -78,6 +78,17 @@
     $(eval _resolved_$(name) := $$(call collapse-pairs,$$(_temp),=))\
 )
 
+$(eval # Implement the legacy behavior when BUILD_BROKEN_DUP_SYSPROP is on.)
+$(eval # Optional assignments are all converted to normal assignments and)
+$(eval # when their duplicates the first one wins)
+$(if $(filter true,$(BUILD_BROKEN_DUP_SYSPROP)),\
+    $(foreach name,$(strip $(4)),\
+        $(eval _temp := $$(subst ?=,=,$$(_resolved_$(name))))\
+        $(eval _resolved_$(name) := $$(call uniq-pairs-by-first-component,$$(_resolved_$(name)),=))\
+    )\
+    $(eval _option := --allow-dup)\
+)
+
 $(2): $(POST_PROCESS_PROPS) $(INTERNAL_BUILD_ID_MAKEFILE) $(API_FINGERPRINT) $(3)
 	$(hide) echo Building $$@
 	$(hide) mkdir -p $$(dir $$@)
@@ -100,7 +111,7 @@
 	        echo "$$(line)" >> $$@;\
 	    )\
 	)
-	$(hide) $(POST_PROCESS_PROPS) $$@ $(5)
+	$(hide) $(POST_PROCESS_PROPS) $$(_option) $$@ $(5)
 	$(hide) echo "# end of file" >> $$@
 endef