align two mirrors to be perfectly parallel and stand between them. any character looking into the recursive reflection is booted out of the plane. add having to be invisible in order to be able to see the recursive focal point for extra kicks.
a pc and one of their mirror duplicates (via plane shift?) has to catch each others' glance in the same mirror at the same time.
slay duplicate, anoint a mirror with the blood of the fallen, mirror then becomes a portal for that person only. except that it works either way... hope the party can tell the two apart....
trick / force a duplicate (or allow it to be a pc, if you are feeling generous) into bull rushing their opposite (or another, if a pc) through a mirror. pity the last person left behind....
charge a mirror with negative or positive energy, and look into it at the same time plane shift is cast. instead of creating a mirror duplicate, it creates an anti-duplicate - as in anti-matter. mutual annihilation can either cast the annihilated one out of the plane, or cast everyone else out of the plane. nobel sacrifice.
perform an action or cause an effect that a mirror cannot duplicate.
go through the back of a mirror, possibly leaving behind a duplicate to terrorize the rest of the party.
crack a mirror knowingly and deliberately accepting 7 years bad luck (curse / eyebite effect?) the crack has to be from one side to the other without actually shattering the mirror.
pcs have to make a rhyme with "mirror, mirror, on the wall..." with the content being something about escaping. each rhyme must be unique, except for the first line. sarcastic entity critiquing their efforts (and possibly allowing diplomacy / bluff / intimidate checks) optional. suggesting that it only speaks in haiku, but only hears couplets.
must accept a quest/geas from a duplicate, something that a duplicate would want.
question: are you using the standardized layout for the mirror plane(s) with constellations? or something completely different?