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This new keyword ability from the Gruul guild in Ravnica Allegiance is bugging me. If I play a creature with Riot, like Rampaging Rendhorn, and I have Rhythm of the Wild in play, it has two instances of Riot. Do I have to choose to put a +1/+1 counter on the creature AND Haste or may I choose 2 instances of the same choice? Like two +1/+1 counters or Haste twice?

You can make any of those combinations of those two choices.

Riot is an ability that creates a replacement effect that modifies how a creature enters the battlefield. As the creature enters the battlefield, you apply each instance of Riot and make your choice of haste or a +1/+1 counter. Those choices are independent, and there is nothing wrong with a creature having two +1/+1 counters or two instances of haste.

A ruling on the card Rhythm of the Wild confirms this:

If a creature enters the battlefield with two instances of riot, you may choose to have it get two +1/+1 counters, one +1/+1 counter and haste, or two instances of haste. Multiple instances of haste on the same creature are redundant, but we’re not going to tell the Gruul how to live their lives.

To clarify the last sentence, haste does not work any differently if you have more than one instance of it, and rule 112.10b says "Effects that remove an ability remove all instances of it." So, if you choose to give the creature haste twice, it's no different than giving it haste once.

Multiple instances of riot are cumulative and independent of each other. The creature enters with any combination of +1/+1 counters and/or instances of haste.

From the wording, riot is a replacement effect that modifies how the permanent it's on enters the battlefield:

614.1c Effects that read “[This permanent] enters the battlefield with . . . ,” “As [this permanent] enters the battlefield . . . ,” or “[This permanent] enters the battlefield as . . . “ are replacement effects.

Multiple replacement effects can be chained on the same original event, as long as the event that results after each step can itself be replaced. Riot works on the creature entering the battlefield, and after it's applied, it's still entering the battlefield, therefore any number of instances of riot can apply to the same creature entering the battlefield:

616.2. A replacement or prevention effect can become applicable to an event as the result of another replacement or prevention effect that modifies the event.
Example: One effect reads “If you would gain life, draw that many cards instead,” and another reads “If you would draw a card, return a card from your graveyard to your hand instead.” Both effects combine (regardless of the order they came into existence): Instead of gaining 1 life, the player puts a card from their graveyard into their hand.

Since each replacement effect is handled separately, for each instance of riot you make a separate, independent choice.

The two instances of riot will be independent of each other, you will make the choice for each instance of Riot, either to give haste or add a +1/+1 counter. Giving haste twice is possible but the second instance of haste will be redundant so you will likely put at least 1 +1/+1 counter on the creature, if you choose to though you can have 2 +1/+1 counters or have the creature have haste, haste.

I can think of a few reasons to avoid putting any counters, though they will be edge cases:

1. 
   Mikaeus, the Unhallowed - If your creature would be haste and undying, you could very well want that first swing to not have the counter on it, either the opponent takes the hit or the creature comes back into play from Undying, now after combat, and you can put the 2 +1/+1 counters on then, for a total of +3/+3.


2. 
   Kulrath Knight - If your opponent has this in play, counters on your creature are a bad idea in general, not being able to attack or block with anything that has a counter of any kind on it makes it hard to win, particularly in the Gruul colors which are usually about getting big things out fast and having them hit hard.

UPDATE

There has been a ruling attached to Rhythm of the Wild dated 1/25/2019(release date):

If a creature enters the battlefield with two instances of riot, you may choose to have it get two +1/+1 counters, one +1/+1 counter and haste, or two instances of haste. Multiple instances of haste on the same creature are redundant, but we’re not going to tell the Gruul how to live their lives.

The correct answer is that we don't know yet. The Comprehensive Rules haven't been updated, since the new set isn't released yet. The rules do say

112.2c ... If an object has multiple instances of the same ability, each instance functions independently. This may or may not produce more effects than a single instance; refer to the specific ability for more information.

Some abilities explicitly specify they're redundant, which is obvious for e.g. flying and shroud. On the other hand, an ability like lifelink that could theoretically work twice like lifelink (you'd gain 2 life for each damage dealt) doesn't, because the rules say it doesn't:

702.15f Multiple instances of lifelink on the same object are redundant.

On the other hand, a lesser known (and more riot-like ability) like amplify is non-redundant:

702.37a Amplify is a static ability. “Amplify N” means “As this object enters the battlefield, reveal any number of cards from your hand that share a creature type with it. This permanent enters the battlefield with N +1/+1 counters on it for each card revealed this way. You can’t reveal this card or any other cards that are entering the battlefield at the same time as this card.”

702.37b If a creature has multiple instances of amplify, each one works separately.

I agree with murgatroid99 that it will likely be cumulative, if so, you can make an different choice for each instance of riot.