Doubles is both a Property and Trait.
While under the effects of this Property, any time two or more Die in your Dice Pool have the same Result, both Dice replace their Results with the maximum that Die can roll.
Example: Getting a Result of 6 on both a D20 and D12 in your Dice Pool would cause the D20's Result to become 20 and the D12's Result to become 12. Furthermore due to the Result of the D20 becoming 20 it also Critically Succeeds.
Please Note: Doubles applies to all Dice in a Dice Pool, not just a single pair. If you had 20 Dice with the same Result, all of them would benefit from Doubles.