diff --git a/lib/storage/metric_name.go b/lib/storage/metric_name.go
index 08a4a755f..bdd7de6d6 100644
--- a/lib/storage/metric_name.go
+++ b/lib/storage/metric_name.go
@@ -367,17 +367,8 @@ func (mn *MetricName) Unmarshal(src []byte) error {
 		}
 	}
 
-	// Verify no identical tag keys.
-	if len(mn.Tags) > 0 {
-		prevKey := mn.Tags[0].Key
-		for i := range mn.Tags[1:] {
-			t := &mn.Tags[1+i]
-			if bytes.Equal(t.Key, prevKey) {
-				return fmt.Errorf("found duplicate key %q", prevKey)
-			}
-			prevKey = t.Key
-		}
-	}
+	// There is no need in verifying for identical tag keys,
+	// since they must be handled in MetricName.Marshal inside marshalTags.
 
 	return nil
 }
@@ -584,8 +575,15 @@ func (ts *canonicalTagsSort) Swap(i, j int) {
 }
 
 func marshalTags(dst []byte, tags []Tag) []byte {
+	var prevKey []byte
 	for i := range tags {
-		dst = tags[i].Marshal(dst)
+		t := &tags[i]
+		if string(prevKey) == string(t.Key) {
+			// Skip duplicate keys, since they aren't allowed in Prometheus data model.
+			continue
+		}
+		prevKey = t.Key
+		dst = t.Marshal(dst)
 	}
 	return dst
 }
diff --git a/lib/storage/metric_name_test.go b/lib/storage/metric_name_test.go
index 65f2100f0..3a7942df8 100644
--- a/lib/storage/metric_name_test.go
+++ b/lib/storage/metric_name_test.go
@@ -34,6 +34,36 @@ func testMetricNameSortTags(t *testing.T, tags, expectedTags []string) {
 	}
 }
 
+func TestMetricNameMarshalDuplicateKeys(t *testing.T) {
+	var mn MetricName
+	mn.AccountID = 123
+	mn.ProjectID = 324
+	mn.MetricGroup = []byte("xxx")
+	mn.AddTag("foo", "bar")
+	mn.AddTag("duplicate", "tag")
+	mn.AddTag("duplicate", "tag")
+	mn.AddTag("tt", "xx")
+	mn.AddTag("duplicate", "tag2")
+
+	var mnExpected MetricName
+	mnExpected.AccountID = 123
+	mnExpected.ProjectID = 324
+	mnExpected.MetricGroup = []byte("xxx")
+	mnExpected.AddTag("duplicate", "tag")
+	mnExpected.AddTag("foo", "bar")
+	mnExpected.AddTag("tt", "xx")
+
+	mn.sortTags()
+	data := mn.Marshal(nil)
+	var mn1 MetricName
+	if err := mn1.Unmarshal(data); err != nil {
+		t.Fatalf("cannot unmarshal mn %s: %s", &mn, err)
+	}
+	if !reflect.DeepEqual(&mnExpected, &mn1) {
+		t.Fatalf("unexpected mn unmarshaled;\ngot\n%+v\nwant\n%+v", &mn1, &mnExpected)
+	}
+}
+
 func TestMetricNameMarshalUnmarshal(t *testing.T) {
 	for i := 0; i < 10; i++ {
 		for tagsCount := 0; tagsCount < 10; tagsCount++ {