Sideboarding can duplicate cards

From time to time I have the issue that drag & drop from sideboard to main board and vice versa creates duplicates of cards. I cannot clearly reproduce this, but it happened multiple times to me.

What I did:
Selected 2+ cards and moved them from side to mainboard multiple times.

Result:
One extra copy spawned in the sideboard → On submit, the game complained about my 16 card sideboard.