prevent the same event being handled twice in the verification request, result in cancellation

src/crypto/verification/request/InRoomChannel.js

@@ -178,6 +178,11 @@ export class InRoomChannel {
      * @returns {Promise} a promise that resolves when any requests as an anwser to the passed-in event are sent.
      */
     async handleEvent(event, request, isLiveEvent) {
+        // prevent processing the same event multiple times, as under
+        // some circumstances Room.timeline can get emitted twice for the same event
+        if (request.hasEventId(event.getId())) {
+            return;
+        }
         const type = InRoomChannel.getEventType(event);
         // do validations that need state (roomId, userId),
         // ignore if invalid

src/crypto/verification/request/VerificationRequest.js

@@ -612,6 +612,20 @@ export class VerificationRequest extends EventEmitter {
         return newRaceIdentifier < oldRaceIdentifier;
     }
 
+    hasEventId(eventId) {
+        for (const event of this._eventsByUs.values()) {
+            if (event.getId() === eventId) {
+                return true;
+            }
+        }
+        for (const event of this._eventsByThem.values()) {
+            if (event.getId() === eventId) {
+                return true;
+            }
+        }
+        return false;
+    }
+
     /**
      * Changes the state of the request and verifier in response to a key verification event.
      * @param {string} type the "symbolic" event type, as returned by the `getEventType` function on the channel.