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.
Maybe it needs to be fixed again Again?
From the articles you list, I think this problem is fixed in SQL Server 2008 R2 SP1.
Since it is not work, you could try to repair the SQL Server Instance.
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