I don't think having Ditto voting against me would be a good idea either.
The townies are probably expecting the Mafia to not vote for the same person because it would be suspiscious...so why not do it anyways. If they truly think about it hard enough they can't base it off whether or not we vote on the same person anyways.
Also Psyduck is more suspiscious than Ditto. We should have Ditto vote against Kadabra because we can easily get away with it and it will put the pressure on Kadabra. Kadabra doesn't handle pressure very well, and doesn't have any actual evidence while we have Butterfree's werewolf death and me cleared of bieng a werewolf.