diff --git a/telethon/network/mtproto_sender.py b/telethon/network/mtproto_sender.py
index 520cadbf..16a82bbd 100644
--- a/telethon/network/mtproto_sender.py
+++ b/telethon/network/mtproto_sender.py
@@ -174,6 +174,15 @@ class MtProtoSender:
         """
 
         # TODO Check salt, session_id and sequence_number
+        if msg_id in self._need_confirmation:
+            # We're yet to acknowledge this message already, so just drop it
+            # as we are already aware of it. TODO Should we force acknowledging
+            # all the self._need_confirmation IDs?
+            self._logger.debug(
+                'Ignoring message pending of acknowledge: {}'.format(msg_id)
+            )
+            return False
+
         self._need_confirmation.append(msg_id)
 
         code = reader.read_int(signed=False)