Can be applied more than once until resistances permanently cap at +100%.
For Example a Proto Shielded enemy with Flesh can have Puncture damage type modifiers increased from from -50% to +100% on shields, an effective 4.0x damage increase, and 0% to +100% on flesh, an effective 2.0x damage increase.
Puncture is already strong against most armor types. +50% damage type modifier to Ferrite Armor, +15% damage type modifier to Alloy Armor, and +25% damage type modifier to Infested Sinew. Since Magus Destruct only applies to the shield and flesh, this can make a total Puncture multiplier of 1.5 × 2.0 = 3x against Ferrite Armored enemies, 1.15 × 2.0 = 2.3x against Alloy Armored enemies, and 1.25 × 2.0 = 2.5x against Infested Sinew Armored enemies, making puncture the highest damage element.