Users get same roles assigned multiple times

We noticed a weird behaviour in our client’s Gateway where one user gets the same role assigned multiple times.
We use the standard User Management component in the project with no additional scripting. There, a checklist is used to assign roles. The problem is that when trying to figure out how many people have that level access, the numbers become greatly exaggerated.
Is this some kind of bug or are we doing something wrong?
We have a development gateway and when we verify everything is OK we export the project and import to live gateway. Could this be causing it?