What happens when a creature gains the Riot keyword ability 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.