I don't patch over patches. When they get to where the patches overlap, I give up on the tube. Aside from that I'll patch the same tube any number of times. I've never had a patch leak either.

I haven't tried it in awhile, but I've never successfully removed one of my patches either. I follow the directions on the patch kit including waiting 5 minutes for the glue to dry before installing the patch. I don't think I could get one off without tearing a big hole in the tube.