>In theory, yes
>In practice : no
>
>see http://stackoverflow.com/questions/39771/is-a-guid-unique-100-of-the-time
>
>and http://blogs.msdn.com/b/oldnewthing/archive/2008/06/27/8659071.aspx

Some unlucky Internet users had that one once in their life. I believe I also had it. Chances like one in 16 trillions or less than being hit by a meteorite happened yesterday. I had two transactions in the same millisecond, which increased the chances of having a duplicate, and I got one.