It still requires to be able to be opened from outside with the key. But you aren’t far off… my door has perimeter sliding bolts that activate when double locking.
The door should be lockable and unlockable from both the inside and the outside, and
If the door is locked from the inside, it should not be unlockable from outside, right?
So you can have a door with a lock on the outside and a separate bolt / latch (or two) on the inside. There is a risk that someone with a duplicate key can lock you in from outside (a common prank) so some versions have a lever on the inside that can unlock the door even if it is locked from outside.
These exist to double lock from inside when going to bed to feel safer. I don’t use this feature but some do, my in-laws for example.
Add a couple of bolts?
It still requires to be able to be opened from outside with the key. But you aren’t far off… my door has perimeter sliding bolts that activate when double locking.
The requirement is that
So you can have a door with a lock on the outside and a separate bolt / latch (or two) on the inside. There is a risk that someone with a duplicate key can lock you in from outside (a common prank) so some versions have a lever on the inside that can unlock the door even if it is locked from outside.