Hello, so it seems in a recent Epic Fight Mod update it conflicts with Kingdom Keys mod,
Didn't have a conflict in: EpicFight-18.0.3-beta (Probably later ones too but I don't have any proof)
Has a conflict in: EpicFight-18.1.18-beta
The conflict happens when in Kingdom Keys you enable an ability (Synch Blade) which allows you to summon one weapon in each hand, without the ability it has no issues.

https://pastebin.com/23D7haTk

Thanks ^^

Fixed in 18.1.19

Could you post the source code of how that skill works?

Sure, essentially to summon a weapon it spawns it in your mainhand, but if the skill is enabled it does that in the offhand too (slot 40).
https://github.com/Wehavecookies56/Kingdom-Keys/blob/master/src/main/java/online/kingdomkeys/kingdomkeys/network/cts/CSSummonKeyblade.java