Independent creation is a valid defense against copyright (as contrasted with patent law where it's not).

If the fix is dead-easy and obvious ("change that = in a if statement to an ==", "add a check for foo", or "make this assignment a 1 instead of a 0"), the GPL license of patch A doesn't prevent anyone from creating (from scratch) patch B that does the same thing. For legal proof purposes, you probably want to ensure that the author of patch B hasn't seen the contents of patch A, though for something as obvious as the cases I present, I'm not even sure I'd worry about that.