diff --git a/src/main/java/dan200/computercraft/shared/network/NetworkHandler.java b/src/main/java/dan200/computercraft/shared/network/NetworkHandler.java
index 5dca012cc..f05f2e35d 100644
--- a/src/main/java/dan200/computercraft/shared/network/NetworkHandler.java
+++ b/src/main/java/dan200/computercraft/shared/network/NetworkHandler.java
@@ -9,6 +9,8 @@ import dan200.computercraft.ComputerCraft;
 import dan200.computercraft.api.ComputerCraftAPI;
 import dan200.computercraft.shared.network.client.*;
 import dan200.computercraft.shared.network.server.*;
+import it.unimi.dsi.fastutil.ints.IntOpenHashSet;
+import it.unimi.dsi.fastutil.ints.IntSet;
 import net.minecraft.entity.player.PlayerEntity;
 import net.minecraft.entity.player.ServerPlayerEntity;
 import net.minecraft.network.PacketBuffer;
@@ -26,7 +28,8 @@ import java.util.function.Function;
 
 public final class NetworkHandler
 {
-    public static SimpleChannel network;
+    private static SimpleChannel network;
+    private static final IntSet usedIds = new IntOpenHashSet();
 
     private NetworkHandler()
     {
@@ -100,6 +103,7 @@ public final class NetworkHandler
      */
     private static <T extends NetworkMessage> void registerMainThread( int id, NetworkDirection direction, Class<T> type, Function<PacketBuffer, T> decoder )
     {
+        if( !usedIds.add( id ) ) throw new IllegalStateException( "Duplicate message kind for for id " + id );
         network.messageBuilder( type, id, direction )
             .encoder( NetworkMessage::toBytes )
             .decoder( decoder )