Which type of index will the optimal (means degrade less) the INSERT
performance, unique or not unique?
The difference if any is negligible.
On the other hand, what could have some impact is the index key length. Smaller is better. You could remove the primary key from the index key and make the index non unique. It will remain being a covering index because all non clustered indexes have the clustered index key on leaf page nodes, assuming your primary key is clustered.
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