Errata

When you have a net advantage from two different sources on a roll (after calculating out any disadvantage), you roll 3d6 instead of 2d6. This is more or less the equivalent of getting a +2 on the roll.

For example, if you're a Nim, you get advantage on Deft from that origin. But if you're a Rogue, you also get advantage on certain actions (such as tinkering or stealth). With doubled advantage, you would now roll 3d6 when taking those actions and take the best result. This means all dice have to be 1 to trigger a roll to confirm a critical.

The reverse is also true for disadvantage. If you have a net disadvantage on a roll, you roll 3d6 and take the worst result. This then triggers a roll to confirm a critical, which is always a 2d6.

See also Collaborating on Rolls for advantage/disadvantage in group action.