Allow duplicate entries after being frozen on the client.

fabric-registry-sync-v0/src/main/java/net/fabricmc/fabric/mixin/registry/sync/SimpleRegistryAccessor.java

@@ -0,0 +1,28 @@
+/*
+ * Copyright (c) 2016, 2017, 2018, 2019 FabricMC
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+package net.fabricmc.fabric.mixin.registry.sync;
+
+import org.spongepowered.asm.mixin.Mixin;
+import org.spongepowered.asm.mixin.gen.Accessor;
+
+import net.minecraft.registry.SimpleRegistry;
+
+@Mixin(SimpleRegistry.class)
+public interface SimpleRegistryAccessor {
+	@Accessor
+	boolean isFrozen();
+}

fabric-registry-sync-v0/src/main/java/net/fabricmc/fabric/mixin/registry/sync/SimpleRegistryMixin.java

@@ -48,13 +48,15 @@
 import org.spongepowered.asm.mixin.injection.callback.CallbackInfoReturnable;
 
 import net.minecraft.registry.MutableRegistry;
+import net.minecraft.registry.Registries;
 import net.minecraft.registry.Registry;
 import net.minecraft.registry.RegistryKey;
 import net.minecraft.registry.SimpleRegistry;
 import net.minecraft.registry.entry.RegistryEntry;
 import net.minecraft.registry.entry.RegistryEntryInfo;
 import net.minecraft.util.Identifier;
 
+import net.fabricmc.api.EnvType;
 import net.fabricmc.fabric.api.event.Event;
 import net.fabricmc.fabric.api.event.EventFactory;
 import net.fabricmc.fabric.api.event.registry.RegistryAttribute;
@@ -66,6 +68,7 @@
 import net.fabricmc.fabric.impl.registry.sync.RemapException;
 import net.fabricmc.fabric.impl.registry.sync.RemapStateImpl;
 import net.fabricmc.fabric.impl.registry.sync.RemappableRegistry;
+import net.fabricmc.loader.api.FabricLoader;
 
 @Mixin(SimpleRegistry.class)
 public abstract class SimpleRegistryMixin<T> implements MutableRegistry<T>, RemappableRegistry, ListenableRegistry<T> {
@@ -74,6 +77,9 @@ public abstract class SimpleRegistryMixin<T> implements MutableRegistry<T>, Rema
 	@Unique
 	private static final Set<String> VANILLA_NAMESPACES = Set.of("minecraft", "brigadier");
 
+	@Unique
+	private static final Logger LOGGER = LoggerFactory.getLogger("FabricRegistrySync");
+
 	@Shadow
 	@Final
 	private ObjectList<RegistryEntry.Reference<T>> rawIdToEntry;
@@ -381,9 +387,16 @@ public void unmap(String name) throws RemapException {
 		}
 	}
 
-	// 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 {
+		// I hate this as much as you do, blame Hypixel for sending duplicate entries to the client via dynamic registries.
+		if (FabricLoader.getInstance().getEnvironmentType() == EnvType.CLIENT
+				&& ((SimpleRegistryAccessor) Registries.REGISTRIES).isFrozen()) {
+			LOGGER.error("Exception caught when adding entry to registry. This is likely a server or mod issue.", t);
+			return t;
+		}
+
+		// Actually throw the exception when a duplicate is found, before the registries are frozen
 		throw t;
 	}
 }

fabric-registry-sync-v0/src/main/resources/fabric-registry-sync-v0.mixins.json

@@ -15,6 +15,7 @@
     "RegistryLoaderMixin",
     "SaveLoadingMixin",
     "SerializableRegistriesMixin",
+    "SimpleRegistryAccessor",
     "SimpleRegistryMixin"
   ],
   "injectors": {