make a "push" on the exit. but it's hard to avoid 100%

You could install Zblock. You could use a trigger_push You could make many teleports. (Like if you have one big, than split it up to more, and than each teleport goes to a diffrent pos.)

Or if you know eventscript; it's possible to make a script, that makes one ghost, if two persons are in each other. But scripting is the hard method.

Triggerpush is a good solution for that. If you surround the teleport destination with triggerpush, the player will be pushed away from the destination before the other player touches it. But don't forget to create some water around the destination too.

You can also create two teleporters. One starts activated, and the other one starts desabled. And when you use them, they will simply toggle.

Why not just make use of info_landmarks. Players will teleport to the new location with the same distance/position to the landmark. So unless two people start off inside each other they will teleport to the new location in the same respective places to the landmarks.

I used that trick to make an endless hallway :P

EDITx2: Here's the page about landmarks.