"gdbus emit" and "gdbus call" each need to explicitly ignore the first (if any) "--" command line argument.  Both use g_option_context_parse to obtain their arguments, and, according to <https://developer.gnome.org/glib/2.48/glib-Commandline-option-parser.html#g-option-context-parse>:  "A '--' option is stripped from argv unless there are unparsed options before and after it, or some of the options after it start with '-'."