Mirage Mirror

If Mirage Mirror becomes a copy of a legendary permanent you control, you'll put one of them into your graveyard.

Once Mirage Mirror's ability resolves, it no longer has that ability.

If an effect begins to apply to Mirage Mirror before it becomes a copy of another permanent, that effect will continue to apply. For example, if Mirage Mirror is activated twice in response to itself targeting first Rampaging Hippo then Frilled Sandwalla, the ability it has while it's a copy of Frilled Sandwalla can be activated and its effect will continue to apply while Mirage Mirror is a copy of Rampaging Hippo.

If you activate Mirage Mirror's ability multiple times in a turn in response to itself, then each time one of those abilities resolves, it will overwrite whatever Mirage Mirror is copying. Mirage Mirror will wind up as a copy of the permanent targeted by the last ability to resolve. When the turn ends, all instances of the ability will wear off at the same time.