Unique, Non-Clustered Index and Unique Clustered Index on Same Column
In SQL Server the PK index will, by default, be a clustered index, containing all of the data rows at the leaf level of the index.

Queries of the form

SELECT COUNT(*) FROM T


or

SELECT ... FROM T ORDER BY ID OFFSET 100 ROWS FETCH NEXT 10 ROWS ONLY


Would normally use the clustered index.  But scanning the whole clustered index to count the rows, or to fetch a page of rows from deep in the sort order is expensive.

So it's a separate non-clustered index on the same key is sometimes created to support these queries.