Can be applied more than once until damage type modifiers permanently cap at +100%.
For example, an enemy with Proto Shield and Flesh can have Heat damage type modifier reduced from -50% to +100% on shields, an effective 4.0x damage increase, and 0% to +100% on flesh, an effective 2.0x damage increase.
Note that Heat is already effective against most flesh types. +25% damage type modifier on Cloned Flesh, +25% damage type modifier on Infested, and +50% damage type modifier on Infested Flesh. Therefore, Magus Accelerant's effectiveness is reduced against such enemies, for Cloned Flesh and Infested only receive up to 2.0 / 1.25 = 1.6x additional damage, and Infested Flesh only receive up to 2.0 / 1.5 = 1.33x additional damage.