They say that imitation is the sincerest form of flattery. While I can understand where this notion comes from, I can't quite say I agree. Most people that have this happen, not only feel less than flattered, but they also feel a little violated.
I usually try to give people the benefit of the doubt and think that if they came up with something around the general time that another person did, it wasn't a case of one copying the other. Often we are influenced by outside sources and what is popular in society at any given time without realizing it. Lets face it there are no new ideas under the sun, there is bound to be overlapping ideas when it comes to artwork. However where there are too many similarities there can be little doubt.
I was rather disappointed when someone directed me to the fact that another artist had copied something I had done. It wasn't a bear, but instead it was some of my online work. I had worked very hard for many days to make something interesting and unique that wasn't the mainstream concept, and I know for an absolute fact my work existed first.
I guess I should try to be happy that they liked what I had done so much they chose to use my concept for their own purposes. Try as hard as I might to feel that way, I find I am having a little trouble with that though...