Counterfeit + igg is not a thing at all. IGG is generally best when you want copper. There's no special advantage from trashing an igg instead of copper. Maybe a third card, like rogue, could make something from it.
Just as a rules clarification, isn't it possible to get two Coppers in your hand by Counterfeiting an IGG? I know three-piling with Duchies is usually the way to go, but that would enable you to get Province with either 3 Copper or 1 IGG + 1 Copper in your remaining hand.
it's certainly possible, but counterfeit and IGG still just equals 5$ (igg, igg, copper, copper, counterfeit) if you choose to take 2 coppers. two igg are 4$ (igg, igg, copper, copper) and you don't lose the igg, which is a good thing. so, since both cards are 5$ and duchies are 5$ too, i doubt you'll ever want to pick counterfeit over igg in an igg mirror... you probably never want to pick up counterfeit in an igg mirror save for edge cases