Is unique index combined with a "not null" constraint equivalent to a PK constraint?
which effectively yields two identical indexes.


The indexes are not identical.  The CI includes all of the other columns, while the PK has only the ItemId key column on the leaf.  So the Clustered Index is probably 100x the size of the non-clustered index.

For instance COUNT(*) and paging queries will be significantly more expensive with only the clustered index, as they will have to use table scans.