Little remark... All non-unique non-clustered indexes are created with a sysem internal unique id, 8 bytes in size, so that sql could differentiate each row. These id "columns" aren't visible or accessible.

If they would have to do anything they could just make this column available for use without modifying. Whatever the column's value, it is good enough for any task (it doesn't have to be "pretty" like a IDENTITY(1,1) column is).