Weakness is not applied correctly on a critical hit.
See Core Rulebook Page 450 under Damage and Core Rulebook page 451 under Doubling and Halving Damage. I'll put the important parts here:
Page 450, Damage:
1. Roll the dice indicated by the weapon, unarmed attack, or spell, and apply the modifiers, bonuses, and penalties that apply to the result of the roll.
2. Determine the damage type.
3. Apply the target’s immunities, weaknesses, and resistances to the damage.
4. If any damage remains, reduce the target’s Hit Points by that amount.
Page 451, Double and Halving Damage (subsection of Step 1)
Sometimes you’ll need to halve or double an amount of damage, such as when the outcome of your Strike is a critical hit, or when you succeed at a basic Reflex save against a spell. When this happens, you roll the damage normally, adding all the normal modifiers, bonuses, and penalties. Then you double or halve the amount as appropriate (rounding down if you halved it).
This means the doubling of damage is only valid to Step 1 in applying damage.
Current Effect in FG: Weakness is applied TWICE on a critical hit.
Expected outcome: Weakness is applied exactly ONCE on a critical hit.
The same applies to Resistances:
Currently: Applied TWICE
Expected: Applied ONCE
WEAK and RESIST effects need to be done at a different point in the calculation.
