Copying a doc with the same doc id does not give an error

Server version: 5.1.0-1544.

1. Load default bucket with 50000 docs
2. Browse to the document editor UI
3. Click on the copy icon for a document.
4. Under the "New Document Key", the doc ID gets filled with <doc_id>_copy.
5. Remove the "_copy", that is, the same ID as the existing doc id.

Actual:
Neither a copy is created with a different name nor an error is thrown.

Expected:
An error should be thrown saying that the doc id exists