"If the clustered index is not created with the UNIQUE property, the Database Engine automatically adds a 4-byte uniqueifier column to the table. When it is required, the Database Engine automatically adds a uniqueifier value to a row to make each key unique. This column and its values are used internally and cannot be seen or accessed by users ."
Msg 666, Level 16, State 2, Line 1
The maximum system-generated unique value for a duplicate group was exceeded for index with partition ID <PARTITIONID>. Dropping and re-creating the index may resolve this; otherwise, use another clustering key.
Important: This is something that is not documented and can change in future versions, so our recommendation is that you should review table design to avoid relying on it.
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