How can I assign the same GTIN to different items with the same UOM?

Summary:

There is a GTIN with PZ as UOM already insered for an item with PZ as UOM from the Item Master Data GTIN section (under Relationship) and the same GTIN has to be filled for another item with the same UOM.
I noticed there are two options while inserting the code: New GTIN and Existing GTIN.
When we try to perform the action with New GTIN there is an error that warns about the fact that the code is already existing in the system.
On the contrary, when we try to perform the Existing GTIN one, by entering exactly the same code, the error message says that the value is not valid.