[1.16.5] java.lang.IllegalArgumentException: Duplicate id value for 16!

I have a modded mob that when I try to spawn it, it won't spawn and a crash happens (doesn't crash whole game though) and it says "Duplicate id value for 16!" and points to the mob's defineSynchedData method. Anyone know what's going on?

Here's the method it was pointing to:

    @Override
    protected void defineSynchedData() {
        super.defineSynchedData();
        this.entityData.define(MY_BOOLEAN_DATA, false);
    }

Edited December 9, 20231 yr by a_random_something