Ensure a unique field in a mixin

Author: ErrorCraft

src/main/java/net/errorcraft/codecium/mixin/minecraft/registry/entry/RegistryEntryListCodecExtender.java

@@ -13,6 +13,7 @@
 import org.spongepowered.asm.mixin.Final;
 import org.spongepowered.asm.mixin.Mixin;
 import org.spongepowered.asm.mixin.Shadow;
+import org.spongepowered.asm.mixin.Unique;
 import org.spongepowered.asm.mixin.injection.At;
 import org.spongepowered.asm.mixin.injection.Inject;
 import org.spongepowered.asm.mixin.injection.ModifyArg;
@@ -26,6 +27,7 @@ public class RegistryEntryListCodecExtender<E> {
     @Final
     private RegistryKey<? extends Registry<E>> registry;
 
+    @Unique
     private static RegistryKey<? extends Registry<?>> tempRegistryKey;
 
     @ModifyArg(