fix the bugs that were causing mutation response errors, and add duplicate subscription checks

MUTATIONS WORK BTW!!!

example/client/index.ts

@@ -16,6 +16,7 @@ const rl = createInterface({
 while (true) {
   const message = await rl.question("Enter a message");
   if (message) {
-    client.sendMutation("createMessage", { room: "1", message: message });
+    const result = await client.sendMutation("createMessage", { room: "1", message: message });
+    console.log("Mutation result", result);
   }
 }