Munchkin

One-shot Items work for a "side" not an individual character. We've ruled for a long time that Mate and Doppleganger do not duplicate one-shot Items. I'm fairly certain that should pop up in a search.

The Doppleganger doubles your combat strength and can only be used when you are alone. Most combat oriented one-shot Items add to your combat strength, so, yes, Doppleganger doubles potions.