Boat Names - Unique ?

[Jd1]

I was doing a web search on a particular boat and found at least three different boats with the same name. I was under the impression that boat names had to be unique. Only one vessel exists with that particular name.
Could someone shed some light on this issue?
I suppose one could cancel the registration on one boat and the name becomes available again ?
How do different countries synchronize their registry or do they ? Could there be a vessel "Example" in each country or would that name be unique in the world?
If the name is not unique, what describes a vessel uniquely?
Any help on clearing this up for me would be appreciated.