diff --git a/fabric-registry-sync-v0/src/main/java/net/fabricmc/fabric/mixin/registry/sync/SimpleRegistryMixin.java b/fabric-registry-sync-v0/src/main/java/net/fabricmc/fabric/mixin/registry/sync/SimpleRegistryMixin.java
index 92ffd5b34..7dbde1ddb 100644
--- a/fabric-registry-sync-v0/src/main/java/net/fabricmc/fabric/mixin/registry/sync/SimpleRegistryMixin.java
+++ b/fabric-registry-sync-v0/src/main/java/net/fabricmc/fabric/mixin/registry/sync/SimpleRegistryMixin.java
@@ -25,6 +25,7 @@ import java.util.Set;
 
 import com.google.common.collect.BiMap;
 import com.google.common.collect.HashBiMap;
+import com.llamalad7.mixinextras.injector.ModifyExpressionValue;
 import com.mojang.serialization.Lifecycle;
 import it.unimi.dsi.fastutil.ints.Int2IntMap;
 import it.unimi.dsi.fastutil.ints.Int2IntOpenHashMap;
@@ -379,4 +380,10 @@ public abstract class SimpleRegistryMixin<T> implements MutableRegistry<T>, Rema
 			fabric_prevEntries = null;
 		}
 	}
+
+	// Actually throw the exception when a duplicate is found.
+	@ModifyExpressionValue(method = "add", at = @At(value = "INVOKE", target = "Lnet/minecraft/util/Util;throwOrPause(Ljava/lang/Throwable;)Ljava/lang/Throwable;"))
+	private <E extends Throwable> E throwOnDuplicate(E t) throws E {
+		throw t;
+	}
 }