model: Prevent duplicate flags when processing update_message_flags.

Previously, the event handler would unconditionally add flags to the message's flag list. If a flag (like 'read') was already present, this created duplicates, which broke subsequent removal operations.

This commit adds a check to ensure the flag is not already present before adding it, making the operation idempotent.

Fixes #1986

Author: sarafarajnasardi

lib/model/message.dart

@@ -736,7 +736,9 @@ class MessageStoreImpl extends HasChannelStore with MessageStore, _OutboxMessage
 
     if (isAdd && (event as UpdateMessageFlagsAddEvent).all) {
       for (final message in messages.values) {
-        message.flags.add(event.flag);
+        if (!message.flags.contains(event.flag)) {
+          message.flags.add(event.flag);
+        }
       }
 
       for (final view in _messageListViews) {
@@ -750,9 +752,13 @@ class MessageStoreImpl extends HasChannelStore with MessageStore, _OutboxMessage
         if (message == null) continue; // a message we don't know about yet
         anyMessageFound = true;
 
-        isAdd
-          ? message.flags.add(event.flag)
-          : message.flags.remove(event.flag);
+        if (isAdd) {
+          if (!message.flags.contains(event.flag)) {
+            message.flags.add(event.flag);
+          }
+        } else {
+          message.flags.remove(event.flag);
+        }
       }
       if (anyMessageFound) {
         // TODO(#818): Support MentionsNarrow live-updates when handling

test/model/message_test.dart

@@ -1702,6 +1702,22 @@ void main() {
         check(store).messages.values
           .single.flags.deepEquals([MessageFlag.starred, MessageFlag.read]);
       });
+
+      test('avoid duplicate flags', () async {
+        await prepare();
+        final message = eg.streamMessage(flags: [MessageFlag.read]);
+        await prepareMessages([message]);
+
+        // Add 'read' flag again
+        await store.handleEvent(mkAddEvent(MessageFlag.read, [message.id]));
+
+        // Should verify no notifications since no actual change happened
+        // But current implementation might notify anyway if it doesn't check for change
+        // optimization. The main point is flags list shouldn't have duplicates.
+        // Let's check the flags first.
+
+        check(store).messages.values.single.flags.deepEquals([MessageFlag.read]);
+      });
     });
 
     group('remove flag', () {