Conditions: multiple applicable instances of advantage/disadvantage.
In a given case that someone would have multiple stacking instances of advantage or disadvantage, each additional instance will provide an extra +2 to the roll.
1. Double advantage: 2d20 +2 +regular modifiers
2. Triple advantage: 2d20+4 +regular modifiers
3. Double disadvantage: 2d20 -2 +regular modifiers
4. Triple disadvantage: 2d20 -4 +regular modifiers