diff --git a/lib/src/services/api/client2client/additional_data.c2c.dart b/lib/src/services/api/client2client/additional_data.c2c.dart
index 02ffb246..5a451bb4 100644
--- a/lib/src/services/api/client2client/additional_data.c2c.dart
+++ b/lib/src/services/api/client2client/additional_data.c2c.dart
@@ -14,6 +14,19 @@ Future<void> handleAdditionalDataMessage(
   Log.info(
     'Got a additional data message: ${message.senderMessageId} from $groupId',
   );
+
+  // Prevent message overwrite: reject if a message with this ID already
+  // exists from a different sender.
+  final existing = await twonlyDB.messagesDao
+      .getMessageById(message.senderMessageId)
+      .getSingleOrNull();
+  if (existing != null && existing.senderId != fromUserId) {
+    Log.warn(
+      '$fromUserId tried to overwrite message from ${existing.senderId}. Dropping.',
+    );
+    return;
+  }
+
   final msg = await twonlyDB.messagesDao.insertMessage(
     MessagesCompanion(
       messageId: Value(message.senderMessageId),
diff --git a/lib/src/services/api/client2client/text_message.c2c.dart b/lib/src/services/api/client2client/text_message.c2c.dart
index 31d225b4..b46e72f2 100644
--- a/lib/src/services/api/client2client/text_message.c2c.dart
+++ b/lib/src/services/api/client2client/text_message.c2c.dart
@@ -16,6 +16,18 @@ Future<void> handleTextMessage(
     'Got a text message: ${textMessage.senderMessageId} from $groupId',
   );
 
+  // Prevent message overwrite: reject if a message with this ID already
+  // exists from a different sender.
+  final existing = await twonlyDB.messagesDao
+      .getMessageById(textMessage.senderMessageId)
+      .getSingleOrNull();
+  if (existing != null && existing.senderId != fromUserId) {
+    Log.warn(
+      '$fromUserId tried to overwrite message from ${existing.senderId}. Dropping.',
+    );
+    return;
+  }
+
   final message = await twonlyDB.messagesDao.insertMessage(
     MessagesCompanion(
       messageId: Value(textMessage.senderMessageId),