The 28-character key is generated from the 16-character key, so two people attempting to register the same 16-key will get the same 28-key.
Once they Key is registered once, it cannot be registered again. However, the 28 character Key is linked to the 16 character Key, so if one's in use by someone, the other will get flagged saying it's in use.
At least that's how I think it works, though that WOULD explain the issues in this thread.
By Siouxdax regarding JSP:
Q u o t e: . . . a forum run by an idiot for idiots . . .