diff --git a/pkg/labels/labels.go b/pkg/labels/labels.go
index 3b462e68d..b919408ed 100644
--- a/pkg/labels/labels.go
+++ b/pkg/labels/labels.go
@@ -200,6 +200,20 @@ func (ls Labels) Has(name string) bool {
 	return false
 }
 
+// HasDuplicateLabels returns whether ls has duplicate label names.
+// It assumes that the labelset is sorted.
+func (ls Labels) HasDuplicateLabelNames() (string, bool) {
+	for i, l := range ls {
+		if i == 0 {
+			continue
+		}
+		if l.Name == ls[i-1].Name {
+			return l.Name, true
+		}
+	}
+	return "", false
+}
+
 // WithoutEmpty returns the labelset without empty labels.
 // May return the same labelset.
 func (ls Labels) WithoutEmpty() Labels {
diff --git a/pkg/labels/labels_test.go b/pkg/labels/labels_test.go
index 173288aff..a8d319c52 100644
--- a/pkg/labels/labels_test.go
+++ b/pkg/labels/labels_test.go
@@ -149,3 +149,29 @@ func TestLabels_MatchLabels(t *testing.T) {
 		testutil.Equals(t, test.expected, got, "unexpected labelset for test case %d", i)
 	}
 }
+
+func TestLabels_HasDuplicateLabelNames(t *testing.T) {
+	cases := []struct {
+		Input     Labels
+		Duplicate bool
+		LabelName string
+	}{
+		{
+			Input:     FromMap(map[string]string{"__name__": "up", "hostname": "localhost"}),
+			Duplicate: false,
+		}, {
+			Input: append(
+				FromMap(map[string]string{"__name__": "up", "hostname": "localhost"}),
+				FromMap(map[string]string{"hostname": "127.0.0.1"})...,
+			),
+			Duplicate: true,
+			LabelName: "hostname",
+		},
+	}
+
+	for i, c := range cases {
+		l, d := c.Input.HasDuplicateLabelNames()
+		testutil.Equals(t, c.Duplicate, d, "test %d: incorrect duplicate bool", i)
+		testutil.Equals(t, c.LabelName, l, "test %d: incorrect label name", i)
+	}
+}
diff --git a/scrape/scrape_test.go b/scrape/scrape_test.go
index 01788bef8..dfde91ddf 100644
--- a/scrape/scrape_test.go
+++ b/scrape/scrape_test.go
@@ -917,7 +917,6 @@ func TestScrapeLoopCacheMemoryExhaustionProtection(t *testing.T) {
 }
 
 func TestScrapeLoopAppend(t *testing.T) {
-
 	tests := []struct {
 		title           string
 		honorLabels     bool
@@ -1535,3 +1534,45 @@ func TestScrapeLoop_DiscardTimestamps(t *testing.T) {
 	}
 	testutil.Equals(t, want, capp.result, "Appended samples not as expected")
 }
+
+func TestScrapeLoopDiscardDuplicateLabels(t *testing.T) {
+	s := teststorage.New(t)
+	defer s.Close()
+
+	app, err := s.Appender()
+	testutil.Ok(t, err)
+
+	ctx, cancel := context.WithCancel(context.Background())
+	sl := newScrapeLoop(ctx,
+		&testScraper{},
+		nil, nil,
+		nopMutator,
+		nopMutator,
+		func() storage.Appender { return app },
+		nil,
+		0,
+		true,
+	)
+	defer cancel()
+
+	// We add a good and a bad metric to check that both are discarded.
+	_, _, _, err = sl.append([]byte("test_metric{le=\"500\"} 1\ntest_metric{le=\"600\",le=\"700\"} 1\n"), "", time.Time{})
+	testutil.NotOk(t, err)
+
+	q, err := s.Querier(ctx, time.Time{}.UnixNano(), 0)
+	testutil.Ok(t, err)
+	series, _, err := q.Select(&storage.SelectParams{}, labels.MustNewMatcher(labels.MatchRegexp, "__name__", ".*"))
+	testutil.Ok(t, err)
+	testutil.Equals(t, false, series.Next(), "series found in tsdb")
+
+	// We add a good metric to check that it is recorded.
+	_, _, _, err = sl.append([]byte("test_metric{le=\"500\"} 1\n"), "", time.Time{})
+	testutil.Ok(t, err)
+
+	q, err = s.Querier(ctx, time.Time{}.UnixNano(), 0)
+	testutil.Ok(t, err)
+	series, _, err = q.Select(&storage.SelectParams{}, labels.MustNewMatcher(labels.MatchEqual, "le", "500"))
+	testutil.Ok(t, err)
+	testutil.Equals(t, true, series.Next(), "series not found in tsdb")
+	testutil.Equals(t, false, series.Next(), "more than one series found in tsdb")
+}
diff --git a/tsdb/head.go b/tsdb/head.go
index 314048248..caebeb4cb 100644
--- a/tsdb/head.go
+++ b/tsdb/head.go
@@ -53,6 +53,10 @@ var (
 	// writable time range.
 	ErrOutOfBounds = errors.New("out of bounds")
 
+	// ErrInvalidSample is returned if an appended sample is not valid and can't
+	// be ingested.
+	ErrInvalidSample = errors.New("invalid sample")
+
 	// emptyTombstoneReader is a no-op Tombstone Reader.
 	// This is used by head to satisfy the Tombstones() function call.
 	emptyTombstoneReader = tombstones.NewMemTombstones()
@@ -921,6 +925,10 @@ func (a *headAppender) Add(lset labels.Labels, t int64, v float64) (uint64, erro
 	// Ensure no empty labels have gotten through.
 	lset = lset.WithoutEmpty()
 
+	if l, dup := lset.HasDuplicateLabelNames(); dup {
+		return 0, errors.Wrap(ErrInvalidSample, fmt.Sprintf(`label name "%s" is not unique`, l))
+	}
+
 	s, created := a.head.getOrCreate(lset.Hash(), lset)
 	if created {
 		a.series = append(a.series, record.RefSeries{
diff --git a/tsdb/head_test.go b/tsdb/head_test.go
index 4b826e50b..b6a7c1cba 100644
--- a/tsdb/head_test.go
+++ b/tsdb/head_test.go
@@ -1255,7 +1255,7 @@ func TestWalRepair_DecodingError(t *testing.T) {
 }
 
 func TestNewWalSegmentOnTruncate(t *testing.T) {
-	dir, err := ioutil.TempDir("", "test_wal_segements")
+	dir, err := ioutil.TempDir("", "test_wal_segments")
 	testutil.Ok(t, err)
 	defer func() {
 		testutil.Ok(t, os.RemoveAll(dir))
@@ -1290,3 +1290,28 @@ func TestNewWalSegmentOnTruncate(t *testing.T) {
 	testutil.Ok(t, err)
 	testutil.Equals(t, 2, last)
 }
+
+func TestAddDuplicateLabelName(t *testing.T) {
+	dir, err := ioutil.TempDir("", "test_duplicate_label_name")
+	testutil.Ok(t, err)
+	defer func() {
+		testutil.Ok(t, os.RemoveAll(dir))
+	}()
+	wlog, err := wal.NewSize(nil, nil, dir, 32768, false)
+	testutil.Ok(t, err)
+
+	h, err := NewHead(nil, nil, wlog, 1000)
+	testutil.Ok(t, err)
+	defer h.Close()
+
+	add := func(labels labels.Labels, labelName string) {
+		app := h.Appender()
+		_, err = app.Add(labels, 0, 0)
+		testutil.NotOk(t, err)
+		testutil.Equals(t, fmt.Sprintf(`label name "%s" is not unique: invalid sample`, labelName), err.Error())
+	}
+
+	add(labels.Labels{{Name: "a", Value: "c"}, {Name: "a", Value: "b"}}, "a")
+	add(labels.Labels{{Name: "a", Value: "c"}, {Name: "a", Value: "c"}}, "a")
+	add(labels.Labels{{Name: "__name__", Value: "up"}, {Name: "job", Value: "prometheus"}, {Name: "le", Value: "500"}, {Name: "le", Value: "400"}, {Name: "unit", Value: "s"}}, "le")
+}