A dispelling weapon functions like a spell storing weapon, but it may only store dispel magic; however, the caster level check to dispel gains an additional bonus equal to the weapon’s enhancement bonus. This bonus also applies to a magus’s dispelling strike arcana or a barbarian’s spell sunder or sunder enchantment combat maneuver check
So how do I get this bonus on my spell sunder check? I already add weapon's enchantment bonus to sunder maneuver, will I get it the second time, only as an untyped one? Should I discharge the stored dispel magic, or is this bonus constant?