Idea for an ultimate: Copycat

I posted something similar long ago but here's another idea for an ultimate ability which I'm sure it could be interesting.

'Copycat'

Select one champion from the enemy team to transform into a 'copy' of his, modifying all your stats to be exactly the same as the target and replacing all your abilities with his own with the exception of your ultimate because 'Copycat' remains in place. However, you deal 2% increased damage to your target to compensate for this. Copycat **lasts indefinitely **and is only put on cooldown once you die or deactivate the ability. During the battle you can reactivate Copycat as a targeted ability on the one you're copying to bring him to a duel; once that happens both you and your copied target are restored to full/equal health, are placed in a mini arena which none of you can leave until one of the two dies and whilst inside it no allies or enemies can enter nor provide benefits or damage either of the two champions. If you manage to kill your copied target you get permanently increased damage output by a small percentage.

Overview of the ability:

This ultimate ability essentially means you can transform into a slightly weaker version of an enemy champion, having the exact same stats as him, both base stats and those from items he currently has scaling accordingly if he buys new items but will not scale down or change until 8 seconds have passed should the copied target sell some of his items and the copying champion will be notified that his target has sold items (this is to prevent a trick in which the copied target would immediately sell all his items to weaken the copying champion during a fight so that his teammates would kill him easily and then 'undo' the sale of his items to get them back) but not his ultimate ability.

This ultimate would most likely be very situational though it could be a powerful tool to deal with situations when one enemy player happens to be 'fed' to the point it's very hard if not impossible to kill him and where essentially the game becomes one sided and no longer fun to play because you're made to feel you no longer have any chance to win. You can use this to effectively eliminate an enemy 'carry' whether he's an ADC or any other class of champion but, since it involves a duel between nearly equally strong champions, it has high stakes and only raw skill will decide the outcome.

What do you think of this?