You guys are going to laugh at this, but I had an odd idea while thinking about multiple clients and cheating.
Instead of just barring anyone from joining twice, what if I allowed for a secondary client, but made it so that they were a dog...? They wouldn't be able to do anything productive (except maybe build), and any demerits they earned would be tied to their main profile, but they'd still be allowed to play. If the first client left, the dog would respawn as a normal player.