How similar is too similar?

When is a design similar enough to your design that you should report it? I don't want to report on someone for no reason, but someone copied an idea of mine in the ULotto contest. I put the words inside a lottery ball, and then someone else put the U in the ball and LOTTO right under it. It looks pretty similar to my design but I don't know if the idea was maybe just obvious enough that it makes sense for someone else to do it. On the other hand, no one else has done it. So I don't really want to report it in case I'm wrong. What should I do?

If you feel the design was a copy, report it and it will go to Design's Court were it will be voted on.