The behavior is actually symmetrical between the player and enemies. Standing in the way of an incoming teleport does 15 internal fire damage and redirects the teleport elsewhere. (The teleporting player/enemy can also take damage if the destination is blocked by something like a barrel.)
Oh, I didn't realize that. Recently I found a demon next to a teleporter with the destination to the tile it was standing on, so when I approached him from the other side, he couldn't help but to teleport endlessly. Obviously I decided to telefrag him, but I got teleported to a random location instead, so I thought the game just checks if the destination tile is occupied and redirects the teleport elsewhere. The damage I did to the demon wasn't enough to kill him, so I though nothing happened.
Still, I would prefer it if telefragging would instagib monsters as in doom and did the random redirect without any damage to the player in case a monster tries to telefrag the player. Maybe it could be exploited by players, but I believe that it's extremely hard/lucky to telefrag an enemy intentionally. Might be as issue in the Spider Lair, dunno.