does it degrade performance on DML statements ?
Is it good to have both indexes on same column ?
In general, it's not a good thing. In case of a clustered index, the index is the data in the table. In case of a nonclustered index, the index contains a copy of the data. Each time you write to the table, you write multiple copies of the data and that can hurt performance.
any advantage on select statements?
Generally speaking very little advantage.
The only advantage could come from the nonclustered index being smaller than the clustered index when a scan is performed. Often, this advantage is eliminated by the subsequent lookups needed to fetch the columns not included in the nonclustered index.
Bottom line: there are some cases where a nonclustered index and a clustered index on the same key are beneficial, but it's very unlikely.
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