Recently, I've been getting some crashes on my server and the console is pointing to an entity from Ancient Spellcraft.

Link to the Crash Log (which doesn't include the name of the entity): https://pastebin.com/8CRLcHFz

Console log (which includes the name):
[12:14:43] [Server thread/ERROR] [FML]: Encountered an exception while constructing entity 'ancientspellcraft:evil_class_wizard'
java.lang.reflect.InvocationTargetException: null
at sun.reflect.NativeConstructorAccessorImpl.newInstance0(Native Method) ~[?:1.8.0_221]
at sun.reflect.NativeConstructorAccessorImpl.newInstance(Unknown Source) ~[?:1.8.0_221]
at sun.reflect.DelegatingConstructorAccessorImpl.newInstance(Unknown Source) ~[?:1.8.0_221]
at java.lang.reflect.Constructor.newInstance(Unknown Source) ~[?:1.8.0_221]
at net.minecraftforge.fml.common.registry.EntityEntryBuilder$ConstructorFactory.apply(EntityEntryBuilder.java:305) [EntityEntryBuilder$ConstructorFactory.class:?]
at net.minecraftforge.fml.common.registry.EntityEntryBuilder$ConstructorFactory.apply(EntityEntryBuilder.java:291) [EntityEntryBuilder$ConstructorFactory.class:?]
at net.minecraftforge.fml.common.registry.EntityEntry.newInstance(EntityEntry.java:68) [EntityEntry.class:?]
at net.minecraft.world.biome.Biome$SpawnListEntry.newInstance(Biome.java:655) [anh$c.class:?]
at net.minecraft.world.WorldEntitySpawner.findChunksForSpawning(WorldEntitySpawner.java:141) [anb.class:?]
at net.minecraft.world.WorldServer.tick(WorldServer.java:203) [oo.class:?]
at net.minecraft.server.MinecraftServer.updateTimeLightAndEntities(MinecraftServer.java:756) [MinecraftServer.class:?]
at net.minecraft.server.dedicated.DedicatedServer.updateTimeLightAndEntities(DedicatedServer.java:397) [nz.class:?]
at net.minecraft.server.MinecraftServer.tick(MinecraftServer.java:668) [MinecraftServer.class:?]
at net.minecraft.server.MinecraftServer.run(MinecraftServer.java:526) [MinecraftServer.class:?]
at java.lang.Thread.run(Unknown Source) [?:1.8.0_221]
Caused by: java.lang.IllegalArgumentException: Duplicate id value for 12!
at net.minecraft.network.datasync.EntityDataManager.register(EntityDataManager.java:106) ~[nb.class:?]
at com.windanesz.ancientspellcraft.entity.living.EntityEvilClassWizard.entityInit(EntityEvilClassWizard.java:60) ~[EntityEvilClassWizard.class:1.12.2-1.2.1]
at net.minecraft.entity.Entity.(Entity.java:217) ~[vg.class:?]
at net.minecraft.entity.EntityLivingBase.(EntityLivingBase.java:164) ~[vp.class:?]
at net.minecraft.entity.EntityLiving.(EntityLiving.java:90) ~[vq.class:?]
at net.minecraft.entity.EntityCreature.(SourceFile:21) ~[vx.class:?]
at net.minecraft.entity.monster.EntityMob.(EntityMob.java:26) ~[ade.class:?]
at electroblob.wizardry.entity.living.EntityEvilWizard.(EntityEvilWizard.java:81) ~[EntityEvilWizard.class:?]
at com.windanesz.ancientspellcraft.entity.living.EntityEvilClassWizard.(EntityEvilClassWizard.java:52) ~[EntityEvilClassWizard.class:1.12.2-1.2.1]
... 15 more
[12:14:43] [Server thread/ERROR] [minecraft/MinecraftServer]: Encountered an unexpected exception
net.minecraft.util.ReportedException: Exception ticking world
at net.minecraft.server.MinecraftServer.updateTimeLightAndEntities(MinecraftServer.java:762) ~[MinecraftServer.class:?]
at net.minecraft.server.dedicated.DedicatedServer.updateTimeLightAndEntities(DedicatedServer.java:397) ~[nz.class:?]
at net.minecraft.server.MinecraftServer.tick(MinecraftServer.java:668) ~[MinecraftServer.class:?]
at net.minecraft.server.MinecraftServer.run(MinecraftServer.java:526) [MinecraftServer.class:?]
at java.lang.Thread.run(Unknown Source) [?:1.8.0_221]
Caused by: java.lang.NullPointerException
at net.minecraft.world.WorldEntitySpawner.findChunksForSpawning(WorldEntitySpawner.java:149) ~[anb.class:?]
at net.minecraft.world.WorldServer.tick(WorldServer.java:203) ~[oo.class:?]
at net.minecraft.server.MinecraftServer.updateTimeLightAndEntities(MinecraftServer.java:756) ~[MinecraftServer.class:?]
... 4 more

It was a duplicate.