I had this problem again, but this time on a box running sql server 2008 R2 SP1 with CU 4.
Backup Encountered exception: 0x80131904 Cannot insert duplicate key row in object 'sys.syscommittab' with unique index 'si_xdes_id'
Failed to flush the commit table to disk in dbid # due to error 2601
This problem is supposed to be fixed, has anyone else had this problem after CU4 on 2008 R2 SP1?
More details about this are in these Knowledge Base Articles.
http://support.microsoft.com/kb/2633146
http://support.microsoft.com/kb/2603910
Maybe it needs to be fixed again Again?