Only add local receipt if it's after existing receipt

Author: andybalaam

src/models/thread.ts

@@ -203,7 +203,16 @@ export class Thread extends ReadReceipt<EmittedEvents, EventHandlerMap> {
     ): void => {
         // Add a synthesized receipt when paginating forward in the timeline
         if (!toStartOfTimeline) {
-            room!.addLocalEchoReceipt(event.getSender()!, event, ReceiptType.Read);
+            const sender = event.getSender();
+            if (sender) {
+                const existingReceipt = this.getReadReceiptForUserId(sender);
+                const existingTimespace = existingReceipt?.data?.ts ?? 0;
+                const recursionSupport =
+                    this.client.canSupport.get(Feature.RelationsRecursion) ?? ServerSupport.Unsupported;
+                if (recursionSupport === ServerSupport.Unsupported && existingTimespace < event.getTs()) {
+                    room!.addLocalEchoReceipt(event.getSender()!, event, ReceiptType.Read);
+                }
+            }
         }
         this.onEcho(event, toStartOfTimeline ?? false);
     };