How unique is the sequential portion of the VIN?

Digits 12 through 17 are the sequential portion of a VIN. These are unique to a vehicle, but can we be more specific? For example, can a Nissan Sentra made in two different plants have the same sequence number? What about from two different model years? Since these attributes (plant, year) are encoded elsewhere in the VIN, it stands to reason there could be two Sentra’s with the same sequence number and their VIN’s still unique. Also, is there a standard around this or could it differ by manufacturer?