diff --git a/pyrogram/session/session.py b/pyrogram/session/session.py
index 6455e958..c156c953 100644
--- a/pyrogram/session/session.py
+++ b/pyrogram/session/session.py
@@ -208,7 +208,9 @@ class Session:
                 MsgId.set_server_time(msg.msg_id / (2 ** 32))
 
             if msg.seq_no % 2 != 0:
-                if msg.msg_id not in self.pending_acks:
+                if msg.msg_id in self.pending_acks:
+                    continue
+                else:
                     self.pending_acks.add(msg.msg_id)
 
             if isinstance(msg.body, (raw.types.MsgDetailedInfo, raw.types.MsgNewDetailedInfo)):