diff --git a/app/vmstorage/main.go b/app/vmstorage/main.go
index 53af4c0e99..0bc3b19f86 100644
--- a/app/vmstorage/main.go
+++ b/app/vmstorage/main.go
@@ -265,6 +265,12 @@ func registerStorageMetrics(strg *storage.Storage) {
 	metrics.NewGauge(`vm_date_metric_ids_search_hits_total`, func() float64 {
 		return float64(idbm().DateMetricIDsSearchHits)
 	})
+	metrics.NewGauge(`vm_index_blocks_with_metric_ids_processed_total`, func() float64 {
+		return float64(idbm().IndexBlocksWithMetricIDsProcessed)
+	})
+	metrics.NewGauge(`vm_index_blocks_with_metric_ids_incorrect_order_total`, func() float64 {
+		return float64(idbm().IndexBlocksWithMetricIDsIncorrectOrder)
+	})
 
 	metrics.NewGauge(`vm_assisted_merges_total{type="storage/small"}`, func() float64 {
 		return float64(tm().SmallAssistedMerges)
diff --git a/lib/storage/index_db.go b/lib/storage/index_db.go
index d5f558845a..8571e36400 100644
--- a/lib/storage/index_db.go
+++ b/lib/storage/index_db.go
@@ -201,6 +201,9 @@ type IndexDBMetrics struct {
 	DateMetricIDsSearchCalls       uint64
 	DateMetricIDsSearchHits        uint64
 
+	IndexBlocksWithMetricIDsProcessed      uint64
+	IndexBlocksWithMetricIDsIncorrectOrder uint64
+
 	mergeset.TableMetrics
 }
 
@@ -235,6 +238,9 @@ func (db *indexDB) UpdateMetrics(m *IndexDBMetrics) {
 	m.DateMetricIDsSearchCalls += atomic.LoadUint64(&db.dateMetricIDsSearchCalls)
 	m.DateMetricIDsSearchHits += atomic.LoadUint64(&db.dateMetricIDsSearchHits)
 
+	m.IndexBlocksWithMetricIDsProcessed = atomic.LoadUint64(&indexBlocksWithMetricIDsProcessed)
+	m.IndexBlocksWithMetricIDsIncorrectOrder = atomic.LoadUint64(&indexBlocksWithMetricIDsIncorrectOrder)
+
 	db.tb.UpdateMetrics(&m.TableMetrics)
 	db.doExtDB(func(extDB *indexDB) {
 		extDB.tb.UpdateMetrics(&m.TableMetrics)
@@ -2391,8 +2397,23 @@ func mergeTagToMetricIDsRows(data []byte, items [][]byte) ([]byte, [][]byte) {
 	if len(tmm.pendingMetricIDs) > 0 {
 		logger.Panicf("BUG: tmm.pendingMetricIDs must be empty at this point; got %d items: %d", len(tmm.pendingMetricIDs), tmm.pendingMetricIDs)
 	}
-	if err := checkItemsSorted(dstItems); err != nil {
-		logger.Errorf("please report this error at https://github.com/VictoriaMetrics/VictoriaMetrics/issues : %s", err)
+	if !checkItemsSorted(dstItems) {
+		// Items could become unsorted if initial items contain duplicate metricIDs:
+		//
+		//   item1: 1, 1, 5
+		//   item2: 1, 4
+		//
+		// Items could become the following after the merge:
+		//
+		//   item1: 1, 5
+		//   item2: 1, 4
+		//
+		// i.e. item1 > item2
+		//
+		// Leave the original items unmerged, so they can be merged next time.
+		// This case should be quite rare - if multiple data points are simultaneously inserted
+		// into the same new time series from multiple concurrent goroutines.
+		atomic.AddUint64(&indexBlocksWithMetricIDsIncorrectOrder, 1)
 		dstData = append(dstData[:0], tmm.dataCopy...)
 		dstItems = dstItems[:0]
 		// tmm.itemsCopy can point to overwritten data, so it must be updated
@@ -2402,26 +2423,30 @@ func mergeTagToMetricIDsRows(data []byte, items [][]byte) ([]byte, [][]byte) {
 			dstItems = append(dstItems, buf[:len(item)])
 			buf = buf[len(item):]
 		}
-		if err := checkItemsSorted(dstItems); err != nil {
-			logger.Panicf("BUG: the original items weren't sorted: %s", err)
+		if !checkItemsSorted(dstItems) {
+			logger.Panicf("BUG: the original items weren't sorted; items=%q", dstItems)
 		}
 	}
 	putTagToMetricIDsRowsMerger(tmm)
+	atomic.AddUint64(&indexBlocksWithMetricIDsProcessed, 1)
 	return dstData, dstItems
 }
 
-func checkItemsSorted(items [][]byte) error {
+var indexBlocksWithMetricIDsIncorrectOrder uint64
+var indexBlocksWithMetricIDsProcessed uint64
+
+func checkItemsSorted(items [][]byte) bool {
 	if len(items) == 0 {
-		return nil
+		return true
 	}
 	prevItem := items[0]
 	for _, currItem := range items[1:] {
 		if string(prevItem) > string(currItem) {
-			return fmt.Errorf("items aren't sorted: prevItem > currItem; prevItem=%X; currItem=%X; items=%X", prevItem, currItem, items)
+			return false
 		}
 		prevItem = currItem
 	}
-	return nil
+	return true
 }
 
 // maxMetricIDsPerRow limits the number of metricIDs in tag->metricIDs row.
diff --git a/lib/storage/index_db_test.go b/lib/storage/index_db_test.go
index 0bd00180bf..ea603cdbe1 100644
--- a/lib/storage/index_db_test.go
+++ b/lib/storage/index_db_test.go
@@ -25,15 +25,15 @@ func TestMergeTagToMetricIDsRows(t *testing.T) {
 			data = append(data, item...)
 			itemsB = append(itemsB, data[len(data)-len(item):])
 		}
-		if err := checkItemsSorted(itemsB); err != nil {
-			t.Fatalf("source items aren't sorted: %s", err)
+		if !checkItemsSorted(itemsB) {
+			t.Fatalf("source items aren't sorted; items:\n%q", itemsB)
 		}
 		resultData, resultItemsB := mergeTagToMetricIDsRows(data, itemsB)
 		if len(resultItemsB) != len(expectedItems) {
 			t.Fatalf("unexpected len(resultItemsB); got %d; want %d", len(resultItemsB), len(expectedItems))
 		}
-		if err := checkItemsSorted(resultItemsB); err != nil {
-			t.Fatalf("result items aren't sorted: %s", err)
+		if !checkItemsSorted(resultItemsB) {
+			t.Fatalf("result items aren't sorted; items:\n%q", resultItemsB)
 		}
 		for i, item := range resultItemsB {
 			if !bytes.HasPrefix(resultData, item) {