On a critical hit where for example you roll 2 d4 instead of 1. Do you add your ability modifier twice (one for each roll) or just add it once to the total of the two rolls?
From PHB p. 196 (emphasis mine),
When you score a critical hit, you get to roll extra dice for the attack’s damage against the target. Roll all of the attack’s damage dice twice and add them together. Then add any relevant modifiers as normal.
For example, if you score a critical hit with a dagger, roll 2d4 ... and then add your relevant ability modifier.
In short, you roll double the number of dice for a crit. However, the static modifiers are only added once.
This also extends to added damage on attacks. The extra dice from a Rogue's sneak attack, Fighter's maneuver dice, or a Paladin's smite (as well as numerous other features) all get doubled.
The static bonuses from Dex, Str, or feats like Sharpshooter (and the like) are only added once.