diff --git a/pkg/channels/wecom/dedupe.go b/pkg/channels/wecom/dedupe.go
index 5f2b4cf81..09f5a8a41 100644
--- a/pkg/channels/wecom/dedupe.go
+++ b/pkg/channels/wecom/dedupe.go
@@ -19,9 +19,9 @@ func markMessageProcessed(msgMu *sync.RWMutex, processedMsgs *map[string]bool, m
 	}
 	(*processedMsgs)[msgID] = true
 
-	// Keep the newest message marker when rotating to bound memory growth.
+	// Keep existing behavior: when over limit, reset dedupe map entirely.
 	if len(*processedMsgs) > maxEntries {
-		*processedMsgs = map[string]bool{msgID: true}
+		*processedMsgs = make(map[string]bool)
 	}
 
 	return true
diff --git a/pkg/channels/wecom_dedupe_test.go b/pkg/channels/wecom_dedupe_test.go
new file mode 100644
index 000000000..d26c3388b
--- /dev/null
+++ b/pkg/channels/wecom_dedupe_test.go
@@ -0,0 +1,42 @@
+package channels
+
+import (
+	"sync"
+	"testing"
+)
+
+func TestMarkMessageProcessed_DuplicateDetection(t *testing.T) {
+	var mu sync.RWMutex
+	processed := make(map[string]bool)
+
+	if ok := markMessageProcessed(&mu, &processed, "msg-1", 1000); !ok {
+		t.Fatalf("first message should be accepted")
+	}
+
+	if ok := markMessageProcessed(&mu, &processed, "msg-1", 1000); ok {
+		t.Fatalf("duplicate message should be rejected")
+	}
+}
+
+func TestMarkMessageProcessed_RotationClearsMapAtBoundary(t *testing.T) {
+	var mu sync.RWMutex
+	processed := make(map[string]bool)
+
+	if ok := markMessageProcessed(&mu, &processed, "msg-1", 1); !ok {
+		t.Fatalf("first message should be accepted")
+	}
+	if len(processed) != 1 {
+		t.Fatalf("expected map size 1 after first insert, got %d", len(processed))
+	}
+
+	// Inserting second unique message exceeds maxEntries and should reset map.
+	if ok := markMessageProcessed(&mu, &processed, "msg-2", 1); !ok {
+		t.Fatalf("second unique message should be accepted")
+	}
+	if len(processed) != 0 {
+		t.Fatalf("expected map to be reset after rotation, got size %d", len(processed))
+	}
+	if processed["msg-2"] {
+		t.Fatalf("expected current message marker to be cleared after rotation")
+	}
+}