diff --git a/pkg/channels/wecom.go b/pkg/channels/wecom.go
index f8daf89de..863efd79a 100644
--- a/pkg/channels/wecom.go
+++ b/pkg/channels/wecom.go
@@ -339,14 +339,15 @@ func (c *WeComBotChannel) processMessage(ctx context.Context, msg WeComBotMessag
 		return
 	}
 	c.processedMsgs[msgID] = true
-	c.msgMu.Unlock()
 
-	// Clean up old messages periodically (keep last 1000)
+	// Clean up old messages periodically (keep last 1000).
+	// Keep the current message id in the new map to preserve dedup behavior.
 	if len(c.processedMsgs) > 1000 {
-		c.msgMu.Lock()
-		c.processedMsgs = make(map[string]bool)
-		c.msgMu.Unlock()
+		c.processedMsgs = map[string]bool{
+			msgID: true,
+		}
 	}
+	c.msgMu.Unlock()
 
 	senderID := msg.From.UserID