[basex-talk] duplicated document in collection

Hello, I am trying BaseX 6.7 and have noticed that the same document can be added to a collection many times using the ADD command (add z.xml), duplicating data wich is not desirable. Is this a bug of this version? In comparison, using Sedna 3.4, the same document can not be added to a collection if it already exists, treating document names as primary key. Is there a solution to this, or is there a way in BaseX 6.7 to have a kind of primary key constraint? Thank you.