First of all you need to register the event as "pre", not "post". Set the last parameter in "RegisterHam" to "false". In "post" you cannot change the damage, as it's already been done. "pre" is called before dealing the damage.
Your arguments in the "Breakable_damage" function are wrong. Take a look at the
API. They should be:
PHP Code:
victim, inflictor, attacker, Float:damage, damage_bits
Now you need to modify the 4th argument (damage). Doubling it should make the entity die twice as fast.
PHP Code:
SetHamParamFloat(4, damage * 2)