When I play multiplayer or when I see myself holding the gun in the third person view, the gun I carry gets duplicated and the player has no custom gun holding animation animations.

I tried reinstalling the mod still, nothing changed.
I'm on 1.12.2 btw.

Any help would be appreciated.

try to change version

Just a mod incompatibility. Update to a modern version (1.18.2 or above) since 1.12.2 is now discontinued.

Aww man... :(