You tell them that you did not order that and you want something the plate you did order the first time.
They shouldn't make you pay extra if they messed up in the first place.
But, you know, I was at a Mexican Restaurant the other day and someone ordered a fajita but got a burrito instead or whatever and the lady got mad at the customer and made a huge scene about it while the customer who got the wrong order remained calm even though she politely asked in the first place that they got her order wrong. The lady who worked there insisted that they pay for both plates.
And you never know: they might do something to your food without you knowing it!