Text (SM): If you tap a permanent for mana, it produces twice as much of that mana instead.
The mana produced is doubled. If you have more than one of these in play, the effect is cumulative. Two gives you 4 times as much, and three gives eight times as much. [Shadowmoor FAQ 2008/04/21]
As an example of doubling, a permanent producing "{W}{U}" would produce "{W}{W}{U}{U}". [Shadowmoor FAQ 2008/04/21]
If another effect would alter the type or amount of mana being produced, you choose the order to apply these effects. See Rule 614. [Shadowmoor FAQ 2008/04/21]
