I do not have the monkey grip plugin and there is nothing equipped in my offhand, sorry for the late replay, I only just now noticed the comment.
Additionally, yes, I do have it set to "<twohanded>" and the attack in question is set to invoke a skill that is using YEP_ActSequence so it can attack all enemies at once but another <twohanded> weapon that doesn't use that skill (instead it's attack hits one target) has the same issue where it seemingly strikes twice.

Here's the other one, still very much the same but it strikes all targets (i've assigned this one to shotguns) & has the action sequence stuff attached (I did this due to a problem i was facing last month where it would individually strike each and every target individually rather than all at once, and as I've stated, its only applied to shotguns)



(i tried adding in some of the extra plugins the DualWield Plugin had compatability, hoping that would resolve the issue, it sadly did not)
Something else i would also like to clarify is that the dual wield weapons are using the same notetag, so its not interchanging notetags with one another, in fact, i had to acutally alter this a while ago and remove a couple of weapons because i couldn't get them to behave like RPG Maker 2003's Weapons nor could I locate a proper solution)