fixits: handle per-rule duplicates

* src/complain.c (duplicate_rule_directive): Here.
* tests/actions.at (Invalid uses of %empty): Check it.

src/complain.c

@@ -421,4 +421,5 @@ duplicate_rule_directive (char const *directive,
   complain_indent (&second, complaint, &i, _("only one %s allowed per rule"), directive);
   i += SUB_INDENT;
   complain_indent (&first, complaint, &i, _("previous declaration"));
+  fixits_register (&second, "");
 }