Game mode: [Singleplayer]
You can attach multiple doors to a single frame.
I just upgraded two doorways from T2 to T3. I tried to replace the doors without removing them first and it just added my T3 doors on top.
I played with it a bit and got four doors on a single frame.
As long as you open the door before trying to place a new one, you can stack one of every type of door.
Steps on how to reproduce issue:
- Open the door.
- Upgrade it.