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The UA Centaur (from Unearthed Arcana: Centaurs and Minotaurs) has the Charge ability:
Charge. If you move at least 20 feet straight toward a target and then hit it with a melee weapon attack on the same turn, roll the weapon’s damage dice twice and add them together. Once you use this ability, you can’t use it again until you finish a short or long rest.
How does this ability interact with a critical hit? Does it triple the weapon damage or quadruple it?
PHB, p. 196:
Roll all of the attack's damage dice twice and add them together.
[...]
To speed up play you can roll all the damage dice at once.
The number of weapon damage dice does not increase for a critical hit, so the additional dice rolls of centaur's Charge ability are the same as for an attack that does not crit.
Rolling "all the damage dice at once" is just a way to speed up play; it's not how the number of damage dice is determined.
