Core:Advantage

Sometimes an effect will specify that you have Advantage on an attack, feat save, resistance save, or some other dice roll involving a single die. When this is the case, you roll that die twice and take the higher of the two rolls. If multiple sources would grant you Advantage, you still only roll the die twice. If you have both Advantage and disadvantage, then you roll normally. This is the case even if you have different numbers of sources of Advantage and Disadvantage.

If you are hidden, then you have Advantage on attack rolls against creatures that can't see you. If you make an attack, then you roll 2d20k1 instead of the normal 1d20. If the creature you're attacking is also hidden, then the Advantage from being hidden and the disadvantage from them being hidden cancel out, resulting in a normal 1d20 roll.