There isn't anything in place right now to prevent players or guild names from being the same. This has already been explained above. It was also said in this thread by @Riverwitch_IV that this concern has been put on GS feedback documents. I can attest to that. And @DSKenshirou_X has also replied and acknowledged it.
The biggest problem comes from what people do with duplicates and the intent behind choosing an identical name.
I would hope that out of decency and respect players would choose to not to create duplicates. Especially not in a manner that is meant to create havoc, trolling or going so far as to disrupt server chats for extended periods of time.
Comments made, responses given. The end.