uno plugin: don't allow g9r9 card specification

uno plugin: don't allow g9r9 card specification

The regexp should make sure that the second card is equal to the first
specified card, instead of matching the card expression one or two
times. Otherwise, a play like 'pl g9r9' would be accepted when the
player had two r9 cards and could play them.

This is only a aesthetical change because the plays would still be
valid, albeit (1) surprising and (2) mismatching what the user actually
wrote.