diff --git a/tsdb/agent/series.go b/tsdb/agent/series.go
index d38518f71..76e734217 100644
--- a/tsdb/agent/series.go
+++ b/tsdb/agent/series.go
@@ -45,14 +45,23 @@ func (m *memSeries) updateTimestamp(newTs int64) bool {
 	return false
 }
 
-// seriesHashmap is a simple hashmap for memSeries by their label set.
-// It is built on top of a regular hashmap and holds a slice of series to
-// resolve hash collisions. Its methods require the hash to be submitted
+// seriesHashmap lets agent find a memSeries by its label set, via a 64-bit hash.
+// There is one map for the common case where the hash value is unique, and a
+// second map for the case that two series have the same hash value.
+// Each series is in only one of the maps. Its methods require the hash to be submitted
 // with the label set to avoid re-computing hash throughout the code.
-type seriesHashmap map[uint64][]*memSeries
+type seriesHashmap struct {
+	unique    map[uint64]*memSeries
+	conflicts map[uint64][]*memSeries
+}
 
-func (m seriesHashmap) Get(hash uint64, lset labels.Labels) *memSeries {
-	for _, s := range m[hash] {
+func (m *seriesHashmap) Get(hash uint64, lset labels.Labels) *memSeries {
+	if s, found := m.unique[hash]; found {
+		if labels.Equal(s.lset, lset) {
+			return s
+		}
+	}
+	for _, s := range m.conflicts[hash] {
 		if labels.Equal(s.lset, lset) {
 			return s
 		}
@@ -60,28 +69,49 @@ func (m seriesHashmap) Get(hash uint64, lset labels.Labels) *memSeries {
 	return nil
 }
 
-func (m seriesHashmap) Set(hash uint64, s *memSeries) {
-	seriesSet := m[hash]
+func (m *seriesHashmap) Set(hash uint64, s *memSeries) {
+	if existing, found := m.unique[hash]; !found || labels.Equal(existing.lset, s.lset) {
+		m.unique[hash] = s
+		return
+	}
+	if m.conflicts == nil {
+		m.conflicts = make(map[uint64][]*memSeries)
+	}
+	seriesSet := m.conflicts[hash]
 	for i, prev := range seriesSet {
 		if labels.Equal(prev.lset, s.lset) {
 			seriesSet[i] = s
 			return
 		}
 	}
-	m[hash] = append(seriesSet, s)
+	m.conflicts[hash] = append(seriesSet, s)
 }
 
-func (m seriesHashmap) Delete(hash uint64, ref chunks.HeadSeriesRef) {
+func (m *seriesHashmap) Delete(hash uint64, ref chunks.HeadSeriesRef) {
 	var rem []*memSeries
-	for _, s := range m[hash] {
-		if s.ref != ref {
-			rem = append(rem, s)
+	unique, found := m.unique[hash]
+	switch {
+	case !found: // Supplied hash is not stored.
+		return
+	case unique.ref == ref:
+		conflicts := m.conflicts[hash]
+		if len(conflicts) == 0 { // Exactly one series with this hash was stored
+			delete(m.unique, hash)
+			return
+		}
+		m.unique[hash] = conflicts[0] // First remaining series goes in 'unique'.
+		rem = conflicts[1:]           // Keep the rest.
+	default: // The series to delete is somewhere in 'conflicts'. Keep all the ones that don't match.
+		for _, s := range m.conflicts[hash] {
+			if s.ref != ref {
+				rem = append(rem, s)
+			}
 		}
 	}
 	if len(rem) == 0 {
-		delete(m, hash)
+		delete(m.conflicts, hash)
 	} else {
-		m[hash] = rem
+		m.conflicts[hash] = rem
 	}
 }
 
@@ -117,7 +147,10 @@ func newStripeSeries(stripeSize int) *stripeSeries {
 		s.series[i] = map[chunks.HeadSeriesRef]*memSeries{}
 	}
 	for i := range s.hashes {
-		s.hashes[i] = seriesHashmap{}
+		s.hashes[i] = seriesHashmap{
+			unique:    map[uint64]*memSeries{},
+			conflicts: nil, // Initialized on demand in set().
+		}
 	}
 	for i := range s.exemplars {
 		s.exemplars[i] = map[chunks.HeadSeriesRef]*exemplar.Exemplar{}
@@ -136,40 +169,49 @@ func (s *stripeSeries) GC(mint int64) map[chunks.HeadSeriesRef]struct{} {
 	defer s.gcMut.Unlock()
 
 	deleted := map[chunks.HeadSeriesRef]struct{}{}
+
+	// For one series, truncate old chunks and check if any chunks left. If not, mark as deleted and collect the ID.
+	check := func(hashLock int, hash uint64, series *memSeries) {
+		series.Lock()
+
+		// Any series that has received a write since mint is still alive.
+		if series.lastTs >= mint {
+			series.Unlock()
+			return
+		}
+
+		// The series is stale. We need to obtain a second lock for the
+		// ref if it's different than the hash lock.
+		refLock := int(series.ref) & (s.size - 1)
+		if hashLock != refLock {
+			s.locks[refLock].Lock()
+		}
+
+		deleted[series.ref] = struct{}{}
+		delete(s.series[refLock], series.ref)
+		s.hashes[hashLock].Delete(hash, series.ref)
+
+		// Since the series is gone, we'll also delete
+		// the latest stored exemplar.
+		delete(s.exemplars[refLock], series.ref)
+
+		if hashLock != refLock {
+			s.locks[refLock].Unlock()
+		}
+		series.Unlock()
+	}
+
 	for hashLock := 0; hashLock < s.size; hashLock++ {
 		s.locks[hashLock].Lock()
 
-		for hash, all := range s.hashes[hashLock] {
+		for hash, all := range s.hashes[hashLock].conflicts {
 			for _, series := range all {
-				series.Lock()
-
-				// Any series that has received a write since mint is still alive.
-				if series.lastTs >= mint {
-					series.Unlock()
-					continue
-				}
-
-				// The series is stale. We need to obtain a second lock for the
-				// ref if it's different than the hash lock.
-				refLock := int(series.ref) & (s.size - 1)
-				if hashLock != refLock {
-					s.locks[refLock].Lock()
-				}
-
-				deleted[series.ref] = struct{}{}
-				delete(s.series[refLock], series.ref)
-				s.hashes[hashLock].Delete(hash, series.ref)
-
-				// Since the series is gone, we'll also delete
-				// the latest stored exemplar.
-				delete(s.exemplars[refLock], series.ref)
-
-				if hashLock != refLock {
-					s.locks[refLock].Unlock()
-				}
-				series.Unlock()
+				check(hashLock, hash, series)
 			}
 		}
+		for hash, series := range s.hashes[hashLock].unique {
+			check(hashLock, hash, series)
+		}
 
 		s.locks[hashLock].Unlock()
 	}
diff --git a/tsdb/agent/series_test.go b/tsdb/agent/series_test.go
index 480f6c026..ae327d858 100644
--- a/tsdb/agent/series_test.go
+++ b/tsdb/agent/series_test.go
@@ -74,3 +74,63 @@ func TestNoDeadlock(t *testing.T) {
 		require.FailNow(t, "deadlock detected")
 	}
 }
+
+func labelsWithHashCollision() (labels.Labels, labels.Labels) {
+	// These two series have the same XXHash; thanks to https://github.com/pstibrany/labels_hash_collisions
+	ls1 := labels.FromStrings("__name__", "metric", "lbl1", "value", "lbl2", "l6CQ5y")
+	ls2 := labels.FromStrings("__name__", "metric", "lbl1", "value", "lbl2", "v7uDlF")
+
+	if ls1.Hash() != ls2.Hash() {
+		// These ones are the same when using -tags stringlabels
+		ls1 = labels.FromStrings("__name__", "metric", "lbl", "HFnEaGl")
+		ls2 = labels.FromStrings("__name__", "metric", "lbl", "RqcXatm")
+	}
+
+	if ls1.Hash() != ls2.Hash() {
+		panic("This code needs to be updated: find new labels with colliding hash values.")
+	}
+
+	return ls1, ls2
+}
+
+// stripeSeriesWithCollidingSeries returns a stripeSeries with two memSeries having the same, colliding, hash.
+func stripeSeriesWithCollidingSeries(_ *testing.T) (*stripeSeries, *memSeries, *memSeries) {
+	lbls1, lbls2 := labelsWithHashCollision()
+	ms1 := memSeries{
+		lset: lbls1,
+	}
+	ms2 := memSeries{
+		lset: lbls2,
+	}
+	hash := lbls1.Hash()
+	s := newStripeSeries(1)
+
+	s.Set(hash, &ms1)
+	s.Set(hash, &ms2)
+
+	return s, &ms1, &ms2
+}
+
+func TestStripeSeries_Get(t *testing.T) {
+	s, ms1, ms2 := stripeSeriesWithCollidingSeries(t)
+	hash := ms1.lset.Hash()
+
+	// Verify that we can get both of the series despite the hash collision
+	got := s.GetByHash(hash, ms1.lset)
+	require.Same(t, ms1, got)
+	got = s.GetByHash(hash, ms2.lset)
+	require.Same(t, ms2, got)
+}
+
+func TestStripeSeries_gc(t *testing.T) {
+	s, ms1, ms2 := stripeSeriesWithCollidingSeries(t)
+	hash := ms1.lset.Hash()
+
+	s.GC(1)
+
+	// Verify that we can get neither ms1 nor ms2 after gc-ing corresponding series
+	got := s.GetByHash(hash, ms1.lset)
+	require.Nil(t, got)
+	got = s.GetByHash(hash, ms2.lset)
+	require.Nil(t, got)
+}