Jeff Moden (7/6/2013)
If there is no unique clustered index, then an 8 byte "uniquefier" is also added to the index.
Not quite
If there's no clustered index, the 8-byte RID is added to all nonclustered indexes as the row location. That's all that's added and it's only when there's no clustered index at all. It's not a uniquifier, it's the physical RID, the combination of File ID, Page Number, Slot index.
Same holds true for when a non-unique clustered index is used except it's worse. In such cases, the columns of the clustered index are added to the columns of the non-clustered index and then the 8 byte uniquifier is added to that which could make a single column clustered index quite wide and comparatively slow.
When there is a clustered index (of any form) then the clustered index key (the entire of it) is added to all nonclustered indexes as the row location. When a clustered index is not unique, the clustered index key gets an additional nullable 4-byte uniquifier added to the clustered index key (only actually has a value on duplicate rows). Since it's part of the clustered index key, the uniquifier is added to all nonclustered indexes as well.
Gail Shaw
Microsoft Certified Master: SQL Server, MVP, M.Sc (Comp Sci)
SQL In The Wild: Discussions on DB performance with occasional diversions into recoverability