diff --git a/cmd/promtool/backfill.go b/cmd/promtool/backfill.go
index 6e7a98c55..b04fb8b55 100644
--- a/cmd/promtool/backfill.go
+++ b/cmd/promtool/backfill.go
@@ -93,7 +93,13 @@ func createBlocks(input []byte, mint, maxt int64, maxSamplesInAppender int, outp
 		nextSampleTs = math.MaxInt64
 
 		err := func() error {
-			w, err := tsdb.NewBlockWriter(log.NewNopLogger(), outputDir, blockDuration)
+			// To prevent races with compaction, a block writer only allows appending samples
+			// that are at most half a block size older than the most recent sample appended so far.
+			// However, in the way we use the block writer here, compaction doesn't happen, while we
+			// also need to append samples throughout the whole block range. To allow that, we
+			// pretend that the block is twice as large here, but only really add sample in the
+			// original interval later.
+			w, err := tsdb.NewBlockWriter(log.NewNopLogger(), outputDir, 2*blockDuration)
 			if err != nil {
 				return errors.Wrap(err, "block writer")
 			}
diff --git a/cmd/promtool/backfill_test.go b/cmd/promtool/backfill_test.go
index 56c3cf70c..05b701a0b 100644
--- a/cmd/promtool/backfill_test.go
+++ b/cmd/promtool/backfill_test.go
@@ -365,6 +365,51 @@ http_requests_total{code="400"} 1 1565166113.989
 				},
 			},
 		},
+		{ // For https://github.com/prometheus/prometheus/issues/8476.
+			ToParse: `# HELP http_requests_total The total number of HTTP requests.
+# TYPE http_requests_total counter
+http_requests_total{code="200"} 1021 0
+http_requests_total{code="200"} 1022 7199
+http_requests_total{code="400"} 1023 0
+http_requests_total{code="400"} 1024 7199
+# EOF
+`,
+			IsOk:                 true,
+			Description:          "One series spanning 2h in same block should not cause problems to other series.",
+			MaxSamplesInAppender: 1,
+			Expected: struct {
+				MinTime   int64
+				MaxTime   int64
+				NumBlocks int
+				Samples   []backfillSample
+			}{
+				MinTime:   0,
+				MaxTime:   7199000,
+				NumBlocks: 1,
+				Samples: []backfillSample{
+					{
+						Timestamp: 0,
+						Value:     1021,
+						Labels:    labels.FromStrings("__name__", "http_requests_total", "code", "200"),
+					},
+					{
+						Timestamp: 7199000,
+						Value:     1022,
+						Labels:    labels.FromStrings("__name__", "http_requests_total", "code", "200"),
+					},
+					{
+						Timestamp: 0,
+						Value:     1023,
+						Labels:    labels.FromStrings("__name__", "http_requests_total", "code", "400"),
+					},
+					{
+						Timestamp: 7199000,
+						Value:     1024,
+						Labels:    labels.FromStrings("__name__", "http_requests_total", "code", "400"),
+					},
+				},
+			},
+		},
 		{
 			ToParse: `no_help_no_type{foo="bar"} 42 6900
 # EOF