Viaweb got copied a lot, and now YC does. I've learned not to worry about it. Clones rarely amount to anything. First of all, anyone who has to copy you is probably not that good. Someone really good wouldn't be able to copy someone else, even if it was to their advantage; they'd be too proud to. Second, people can only copy what's visible. They can't copy the half of the project that's in your head, and that is probably the more valuable half.