In MTG, if you have a Renegade Doppelganger on the battlefield and you put a Halimar Excavator into play...?

If you have a Renegade Doppelganger on the battlefield and you put a Halimar Excavator into play, how many cards will you make the target player put into the graveyard (2 or 4) and why?In MTG, if you have a Renegade Doppelganger on the battlefield and you put a Halimar Excavator into play...?
1 or 2 depending on how you stack the triggers. When the Halimar Excavator enters the battlefield it's enter the battlefield trigger and Renegade Doppelganger's enter the battlefield trigger go on the stack, you choose the order that they happen in because you control both effects. If you let the doppelganger's ability resolve first and then the Halimar Excavator you will mill 2 cards because you control two allies. If you let the Halimar Excavator's ability resolve first and then the doppleganger you'll only mill 1 card.

You can't mill 4 cards because when your Halimar Excavator entering the battlefield triggers the Renegade doppleganger it will become a copy of Halimar Excavator when it's already in play and won't trigger.In MTG, if you have a Renegade Doppelganger on the battlefield and you put a Halimar Excavator into play...?
Previous answer is incorrect.

If you have a Renegade Doppleganger already in play on the battlefield, and a Halimar Excavator comes into play, your opponent will put the top 4 cards of their library in the graveyard. Halimar excavator checks for another creature to resolve, and then becomes the creature until end of turn. Once Halimar Excavator resolves, it triggers the Doppleganer's effect, thus making 2 allies at the same time it enters the battlefield.

The reason it causes 4 to mill and not two, is because they are both now ally creatures with "When Halimar Excavator or another ally enters the battlefield under your control, target player puts top X cards of his or her library into the graveyard, where X is the number of allies you control". When Excavator resolves, you suddenly have 2. Each one mills for 2. Hope this helps, I just constructed a deck around that same strategy, enjoy.