Fixed error message (Issue #380)

lark/load_grammar.py

@@ -538,7 +538,7 @@ class Grammar:
             for dups in duplicates.values():
                 if len(dups) > 1:
                     if dups[0].expansion:
-                        raise GrammarError("Rules defined twice: %s" % ', '.join(str(i) for i in duplicates))
+                        raise GrammarError("Rules defined twice: %s\n\n(Might happen due to colliding expansion of optionals: [] or ?)" % ''.join('\n  * %s' % i for i in dups))
 
                     # Empty rule; assert all other attributes are equal
                     assert len({(r.alias, r.order, r.options) for r in dups}) == len(dups)