I've had someone copy my art before. What I think is the most disturbing part about it is that everyone takes their word for it even after you come on the scene to clear up things with the truth. It's like now YOU have to prove you're the original artist and you weren't copying THEM.
Fortunately, I had all the proof I needed and even the date I had drawn it was on the picture. Even after having been caught red handed, however, this person showed no remorse and we were in conflict for a time after that.
Not a big art guy, but I report plagerism when i find it to the original artist. Its also very amusing to see how put out people can be for their hard work when they get busted. I had one web designer who had stolen kyoht and goldenwolfen's artwork and blatantly redone it for a website they got paid for tell me "I have four children how can you be so cruel." Well maybe this will help you teach your four kids that stealing art is still stealing.