diff --git a/lib/storage/block_stream_writer.go b/lib/storage/block_stream_writer.go
index 899facbf7..f80157b6b 100644
--- a/lib/storage/block_stream_writer.go
+++ b/lib/storage/block_stream_writer.go
@@ -184,9 +184,11 @@ func (bsw *blockStreamWriter) MustClose() {
 }
 
 // WriteExternalBlock writes b to bsw and updates ph and rowsMerged.
-func (bsw *blockStreamWriter) WriteExternalBlock(b *Block, ph *partHeader, rowsMerged *uint64) {
+func (bsw *blockStreamWriter) WriteExternalBlock(b *Block, ph *partHeader, rowsMerged *uint64, needDedup bool) {
 	atomic.AddUint64(rowsMerged, uint64(b.rowsCount()))
-	b.deduplicateSamplesDuringMerge()
+	if needDedup {
+		b.deduplicateSamplesDuringMerge()
+	}
 	headerData, timestampsData, valuesData := b.MarshalData(bsw.timestampsBlockOffset, bsw.valuesBlockOffset)
 	usePrevTimestamps := len(bsw.prevTimestampsData) > 0 && bytes.Equal(timestampsData, bsw.prevTimestampsData)
 	if usePrevTimestamps {
diff --git a/lib/storage/block_stream_writer_timing_test.go b/lib/storage/block_stream_writer_timing_test.go
index 8e51d06a2..a8b2c0d06 100644
--- a/lib/storage/block_stream_writer_timing_test.go
+++ b/lib/storage/block_stream_writer_timing_test.go
@@ -49,7 +49,7 @@ func benchmarkBlockStreamWriter(b *testing.B, ebs []Block, rowsCount int, writeR
 
 			bsw.InitFromInmemoryPart(&mp)
 			for i := range ebsCopy {
-				bsw.WriteExternalBlock(&ebsCopy[i], &ph, &rowsMerged)
+				bsw.WriteExternalBlock(&ebsCopy[i], &ph, &rowsMerged, false)
 			}
 			bsw.MustClose()
 			mp.Reset()
diff --git a/lib/storage/merge.go b/lib/storage/merge.go
index 073361064..2795b2439 100644
--- a/lib/storage/merge.go
+++ b/lib/storage/merge.go
@@ -76,14 +76,14 @@ func mergeBlockStreamsInternal(ph *partHeader, bsw *blockStreamWriter, bsm *bloc
 			if bsm.Block.bh.TSID.Less(&pendingBlock.bh.TSID) {
 				logger.Panicf("BUG: the next TSID=%+v is smaller than the current TSID=%+v", &bsm.Block.bh.TSID, &pendingBlock.bh.TSID)
 			}
-			bsw.WriteExternalBlock(pendingBlock, ph, rowsMerged)
+			bsw.WriteExternalBlock(pendingBlock, ph, rowsMerged, true)
 			pendingBlock.CopyFrom(bsm.Block)
 			continue
 		}
 		if pendingBlock.tooBig() && pendingBlock.bh.MaxTimestamp <= bsm.Block.bh.MinTimestamp {
 			// Fast path - pendingBlock is too big and it doesn't overlap with bsm.Block.
 			// Write the pendingBlock and then deal with bsm.Block.
-			bsw.WriteExternalBlock(pendingBlock, ph, rowsMerged)
+			bsw.WriteExternalBlock(pendingBlock, ph, rowsMerged, true)
 			pendingBlock.CopyFrom(bsm.Block)
 			continue
 		}
@@ -119,13 +119,13 @@ func mergeBlockStreamsInternal(ph *partHeader, bsw *blockStreamWriter, bsm *bloc
 		tmpBlock.timestamps = tmpBlock.timestamps[:maxRowsPerBlock]
 		tmpBlock.values = tmpBlock.values[:maxRowsPerBlock]
 		tmpBlock.fixupTimestamps()
-		bsw.WriteExternalBlock(tmpBlock, ph, rowsMerged)
+		bsw.WriteExternalBlock(tmpBlock, ph, rowsMerged, true)
 	}
 	if err := bsm.Error(); err != nil {
 		return fmt.Errorf("cannot read block to be merged: %w", err)
 	}
 	if !pendingBlockIsEmpty {
-		bsw.WriteExternalBlock(pendingBlock, ph, rowsMerged)
+		bsw.WriteExternalBlock(pendingBlock, ph, rowsMerged, true)
 	}
 	return nil
 }
diff --git a/lib/storage/raw_row.go b/lib/storage/raw_row.go
index b1d978d33..e3dcdfc9f 100644
--- a/lib/storage/raw_row.go
+++ b/lib/storage/raw_row.go
@@ -115,7 +115,7 @@ func (rrm *rawRowsMarshaler) marshalToInmemoryPart(mp *inmemoryPart, rows []rawR
 
 		rrm.auxValues, scale = decimal.AppendFloatToDecimal(rrm.auxValues[:0], rrm.auxFloatValues)
 		tmpBlock.Init(tsid, rrm.auxTimestamps, rrm.auxValues, scale, precisionBits)
-		rrm.bsw.WriteExternalBlock(tmpBlock, ph, &rowsMerged)
+		rrm.bsw.WriteExternalBlock(tmpBlock, ph, &rowsMerged, false)
 
 		tsid = &r.TSID
 		precisionBits = r.PrecisionBits
@@ -125,7 +125,7 @@ func (rrm *rawRowsMarshaler) marshalToInmemoryPart(mp *inmemoryPart, rows []rawR
 
 	rrm.auxValues, scale = decimal.AppendFloatToDecimal(rrm.auxValues[:0], rrm.auxFloatValues)
 	tmpBlock.Init(tsid, rrm.auxTimestamps, rrm.auxValues, scale, precisionBits)
-	rrm.bsw.WriteExternalBlock(tmpBlock, ph, &rowsMerged)
+	rrm.bsw.WriteExternalBlock(tmpBlock, ph, &rowsMerged, false)
 	if rowsMerged != uint64(len(rows)) {
 		logger.Panicf("BUG: unexpected rowsMerged; got %d; want %d", rowsMerged, len(rows))
 	}