Passive RFID Tag: Unique Identifiers
No, they're not globally unique, they're unique per manufacturer. For one thing, not all manufacturers use the same code length. If I want to start producing RFID tags tomorrow I can do whatever I please to do, like producing two RFIDs for every code, or even produce a whole batch with the same code. If you want to have them unique, you'll have to stick to one manufacturer.
That said, the codes are pretty long, so chances are slim that you'll have two colliding codes.  

In the (near?) future RFIDs are going to replace bar codes in logistics. Those will have a combination of a non-unique product code, like EPC (European Product Code) and a unique ID.