A Guid is 32 hex digits grouped into chunks of 8-4-4-4-12. Number of unique Guids is 2^128. Thats a lot..
Guids are created in different ways. The most used Guid is a version 4 Guid based on Time, MAC-address and Random data. (the first number of 3rd chunk is the version number)
version
|
542DF77C-B284-4623-9BC8-FD1E9343CD2C
1A32EAFB-28DD-4E4A-AD27-58B9DD301CD9
87FD5B4E-E733-4B18-951E-F6EBC8960D22
DCE7501C-D419-424A-8C5B-FBE2589AFB73
B056348F-3E4A-4739-87A9-7872FB083295
Since it's using both Mac-address and Time, the only way to create a duplicate Guid is to adjust them clock on your server back in time and try to regenerate a duplicate. You'll never manage to create duplicates, but i theory you can.