When doing mass-change on multiple artifacts with an open list fields, if in this field we add a non existing value (a new open value), this value will be added as many times there is artifacts. For each updated artifacts, the new open value will be inserted in database without checking if it already exists or not resulting in many duplication.

As only Tracker administrator can do artifact mass-change and it happens only when we add new value to an open list field, the scenario is rare. But when we do mass-change it can be on many (many many) artifacts leading to a lot of duplication.