If you're getting that error, you're either trying to insert a row that already exists or there are duplicates in the set you're inserting. There's no other reasons you'd be getting a pk violation.

Try using a NOT EXISTS instead of the join. Should be equivalent, but a not exists is harder to mes up (eg if the column you're checking for null is nullable itself)

Check this to see if there are dups in the source data:

SELECT

u.SR_ID

,u.IR_ID

,u.Ordernr

,count(*)

FROM #Usage u

group by u.SR_ID

,u.IR_ID

,u.Ordernr

order by count(*) desc

What are the data types involved?