Reproduce:

1. create a item with a statement, add two same references to it (e.g. https://test.wikidata.org/wiki/Q97744).
2. create another item with same statement without references (e.g. https://test.wikidata.org/wiki/Q53752).
3. try to merge the first item to the second one.

Expected:
The duplicated references will be discarded.

Actual:
Error "The statement has already a reference with hash ###"

Note merging an item with two identical statements with common references (e.g. https://test.wikidata.org/wiki/Q150012) also fails.