Added check for duplicated chunks during downsampling.

Signed-off-by: Bartlomiej Plotka <[email protected]>

pkg/compact/downsample/downsample.go

@@ -116,6 +116,15 @@ func Downsample(
 		if err := indexr.Series(postings.At(), &lset, &chks); err != nil {
 			return id, errors.Wrapf(err, "get series %d", postings.At())
 		}
+
+		if len(chks) > 1 {
+			for i, c := range chks[1:] {
+				if chks[i].MaxTime >= c.MinTime {
+					return id, errors.Errorf("found overlapping chunks within series %d. Chunks expected to be ordered by min time and non-overlapping, got: %v", postings.At(), chks)
+				}
+			}
+		}
+
 		// While #183 exists, we sanitize the chunks we retrieved from the block
 		// before retrieving their samples.
 		for i, c := range chks {