Text (8th/9th/10): If a source would deal damage to a creature or player, it deals double that damage to that creature or player instead.
It is a replacement effect (see Rule 614) applied when damage is would be dealt. [D'Angelo 1999/06/01]
If a spell or ability damages multiple things, divide up the damage before applying this effect. [WotC Rules Team 1998/10/18] This means you cannot normally end up with an odd amount of damage on something.
If you have two of these in play, the damage is multiplied by 4. [8E FAQ 2003/07/28]
The multiplied damage counts in all ways as if it came from the original source. Furnace of Rath is not the source. [8E FAQ 2003/07/28]
The trample rules cause damage to be divided before it is doubled. [D'Angelo 1999/06/01]