Renegade Doppelganger

If Renegade Doppelganger becomes a copy of a legendary creature, you will have to put one of them into the graveyard as a state-based action.

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.

If Renegade Doppelganger becomes a copy of a token, Renegade Doppelganger copies the original characteristics of that token as stated by the effect that put it onto the battlefield. Renegade Doppelganger doesn't become a token.

When a creature enters under your control, you can choose not to copy it. In that case, Renegade Doppelganger remains 0/1 and still has its triggered ability.

If Renegade Doppelganger becomes a copy of another creature, that doesn't count as having a creature enter. Renegade Doppelganger was already on the battlefield; it only changed its characteristics. If Renegade Doppelganger gains any "enters" triggered abilities, they won't do anything. The same is true of any "as [this creature] enters" or "[this creature] enters with" abilities.

Noncopy effects that have already applied to Renegade Doppelganger will continue to apply to it after it becomes a copy of something else. For example, if Giant Growth had given it +3/+3 earlier in the turn, then it becomes a copy of Runeclaw Bear (a 2/2 creature), it will be a 5/5 Runeclaw Bear.