Don't duplicate entries in MOZCONFIG_MODULES (bug 304504). r=bsmedberg.

client.mk

@@ -346,7 +346,7 @@ MOZCONFIG_MODULES += $(foreach project,$(MOZ_PROJECT_LIST),$(BOOTSTRAP_$(project
 # Using $(sort) here because it also removes duplicate entries.
 MOZ_MODULE_LIST := $(sort $(MOZ_MODULE_LIST))
 LOCALE_DIRS := $(sort $(LOCALE_DIRS))
-MOZCONFIG_MODULES += $(sort $(MOZCONFIG_MODULES))
+MOZCONFIG_MODULES := $(sort $(MOZCONFIG_MODULES))
 
 # Change CVS flags if anonymous root is requested
 ifdef MOZ_CO_USE_MIRROR