Multiplying Damage
Sometimes you multiply your damage by some factor, such as on a critical hit (see page 245). In this case, you do not literally multiply your damage roll result by that factor. Instead, you roll the damage (adding all modifiers) the number of times specified and total the results. If you multiply damage more than once, each multiplier applies to the original, unmultiplied damage. Thus, doubling damage twice is equivalent to rolling the damage (adding all modifiers) three times—once for the original damage and once for each doubling.
Source
Core Rulebook pg. 241