The add and duplicate functionality (used when adding items) duplicates most of the fields in the window in order to reduce cataloging time. With autobarcode disabled the barcode field is NOT duplicated. With autobarcode enabled the barcode field IS duplicated. The proper function is to NOT duplicate the barcode. (tried on 3.8.5)
Still valid in current master, the duplicate behaviour changes with the autobarcode setting - I think it should not duplicate the barcode, but leave it empty, or better: generate the next.
I've just tested this on master duplicating an existing item and using the duplicate fix of bug 23851 - in both cases, the barcode was left empty.