What would happen if a two-door (2 way) teleportation device were placed into its reciprocal doorway?

hmmm, this question is pretty good. I get what your saying. If you have 2 doors, and place one in the other.....hmmm.....I honestly don't know, maybe the 1st door going into the other will disappear, since it has no place to re-emerge.....it will be forever stuck in the space-time continuum.