If Renegade Doppelganger becomes a copy of a creature with an ability that causes you to make a choice (such as naming a card or choosing a color) as it enters, and another ability that refers to that choice, those abilities of the Doppelganger won't do anything. You won't have the chance to make a choice since the Doppelganger is already on the battlefield, so the value of that choice is undefined.