Use target_counter. Target the triggers (or func_buttons if you prefer) to the target_counter instead of the door. Set the count value in the counter properties to correspond to the amount of trigger you want to use.
Now, the target_counter is a bit tricky entity. In theory it should work if you just target it to the door. However, you may find it's not so easily obedient. Thus, you can do a small trick, and target the counter to a trigger located inside a structure or some other place not accessible to the player. Then target this "extra" trigger to the door.
Frozen by ICARUS