I use SDDM with KDE Plasma 5, and in my login screen every user is displayed twice. I'm talking about userland users, not system users.

I've also tried to create ex novo a new test user, just to see the outcome, and also this one is displayed twice (so I'm sure that this is nor related to current users' configuration).

It is more an annoyance than a real issue, but what can be the cause of this? And, can this behavior be prevented?