I believe this is a bug:
It is allowed to enter a Goods Receipt with item serial number which has already been used before, but currently has zero qty in stock.
If we try to return goods from delivery which has been entered before the goods receipt, then we get an error RecordCheckError, vc: PUVc field: SerialNr record 2017061 error: 1243 text: "Serial Number already registered"

v. 8.5 2023-02-08 (build 85510137)