Real copycats exist and should be dealt with but be careful it doesn't turn into a witch-hunt. I already had some people accusing/listing me as copycat/thief, according to them I had stolen my models from a popular game. The realty however is that multiple game makers license and use my work in their games, so obviously my work appears in games.
Also artists/sellers that operate under different usernames on different marketplaces have gotten accused as thief's (sometimes they are but often they are not).
Every case should be thoroughly investigated (preferably not by a group of outraged individuals).