Is it needed to add additional column to make this clustered index unique?
Should I add ProductID also to make the clustered index unique?


You should delete cidx_RequistionProducts and make the primary key clustered instead. The index that is created to support the primary key will do the job for you when filtering on RequisitionID.


  What are the pros and cons?


I see no benefit of having two indexes when one is enough to get the job done. Removing the extra index will save you disk space and IO cost when updating the table.