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Randomly copies a move from an opposing Pokemon's current moveset, even if the user has the copied move in its current moveset. The move copied replaces Mimic but retains Mimic's current PP (but not its maximum PP, this will sometimes cause current PP to exceed maximum PP). Switching out will change the move back to Mimic.
Mimic has little competitive use. It's sometimes used by Pokemon as Jynx who lack better coverage moves and can copy a useful move such as Recover from Starmie, but it's unreliable and it needs luck to work. Often it's not worth the moveslot.