bridge: a discovery request that arrives twice is answered once

A broker that mirrors another delivers every message twice, and the bridge
answered both copies of a discovery request. A request is now remembered
by its messageId for 60 s, as a directive is, and its copy is dropped with
a log line. The messageId is read from the header of the Discover
directive or from the request itself when that is the header alone.
A request without a messageId is answered every time, as before.

Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>

src/Alex2Node.ts

@@ -50,6 +50,20 @@ const DESCRIBED = [
   "customIdentifier", "cookie",
 ];
 
+// The messageId of a discovery request: in the header of the Discover directive, or beside the namespace and the
+// name when the request is the header alone. undefined for a request without one, which is answered every time.
+function requestId(text: string): string | undefined {
+  let request: { directive?: { header?: unknown }; header?: unknown } | null;
+  try {
+    request = JSON.parse(text);
+  } catch {
+    return undefined;
+  }
+  const header = request?.directive?.header ?? request?.header ?? request;
+  const { messageId } = (typeof header === "object" && header !== null ? header : {}) as { messageId?: unknown };
+  return typeof messageId === "string" ? messageId : undefined;
+}
+
 /**
  * The Alexa-to-MQTT bridge: one broker connection for a user's root topic, a set of registered devices, discovery and
  * directive dispatch.
@@ -160,6 +174,11 @@ class Alex2MQTT extends EventEmitter {
     this.log(`MQTT Message Received`, { topic, payload: text });
     try {
       if (topic === topics.discover(this.rootTopic)) {
+        const messageId = requestId(text);
+        if (messageId !== undefined && this.dispatcher.arrivedBefore("", messageId)) {
+          this.log(`the discovery request ${messageId} is dropped: it arrived before`);
+          return;
+        }
         await this.answerDiscovery();
         return;
       }

src/dispatcher.ts

@@ -240,6 +240,18 @@ export class Dispatcher {
     this.options.log(`warning: Amazon documents no DeferredResponse for ${namespace}, Alexa may not wait for the answer`);
   }
 
+  /**
+   * true for a message that came before, within REMEMBERED_MS: a broker that mirrors another delivers each message
+   * twice. endpointId is "" for a message to the root, a discovery request.
+   */
+  arrivedBefore(endpointId: string, messageId: string, now = Date.now()): boolean {
+    this.forget(now);
+    const id = key(endpointId, messageId);
+    if (this.arrived.has(id)) return true;
+    this.arrived.set(id, now);
+    return false;
+  }
+
   /** The text of a message on the directive topic of endpointId. Resolves when the handlers are done. */
   async dispatch(endpointId: string, text: string): Promise<void> {
     const { log, emit } = this.options;
@@ -258,13 +270,9 @@ export class Dispatcher {
     const { namespace, name, messageId } = raw.header;
     const correlationToken = typeof raw.header.correlationToken === "string" ? raw.header.correlationToken : "";
     const receivedAt = Date.now();
-    this.forget(receivedAt);
-    if (typeof messageId === "string") {
-      if (this.arrived.has(key(endpointId, messageId))) {
-        log(`the directive ${messageId} for ${endpointId} is dropped: it arrived before`);
-        return;
-      }
-      this.arrived.set(key(endpointId, messageId), receivedAt);
+    if (typeof messageId === "string" && this.arrivedBefore(endpointId, messageId, receivedAt)) {
+      log(`the directive ${messageId} for ${endpointId} is dropped: it arrived before`);
+      return;
     }
 
     const device = this.options.getDevice(endpointId);