diff --git a/tsdb/head_bench_test.go b/tsdb/head_bench_test.go
index a03794810..51de50ec2 100644
--- a/tsdb/head_bench_test.go
+++ b/tsdb/head_bench_test.go
@@ -36,7 +36,7 @@ func BenchmarkHeadStripeSeriesCreate(b *testing.B) {
 	defer h.Close()
 
 	for i := 0; i < b.N; i++ {
-		h.getOrCreate(uint64(i), labels.FromStrings("a", strconv.Itoa(i)))
+		h.getOrCreate(uint64(i), labels.FromStrings(labels.MetricName, "test", "a", strconv.Itoa(i), "b", strconv.Itoa(i%10), "c", strconv.Itoa(i%100), "d", strconv.Itoa(i/2), "e", strconv.Itoa(i/4)))
 	}
 }
 
@@ -54,8 +54,8 @@ func BenchmarkHeadStripeSeriesCreateParallel(b *testing.B) {
 
 	b.RunParallel(func(pb *testing.PB) {
 		for pb.Next() {
-			i := count.Inc()
-			h.getOrCreate(uint64(i), labels.FromStrings("a", strconv.Itoa(int(i))))
+			i := int(count.Inc())
+			h.getOrCreate(uint64(i), labels.FromStrings(labels.MetricName, "test", "a", strconv.Itoa(i), "b", strconv.Itoa(i%10), "c", strconv.Itoa(i%100), "d", strconv.Itoa(i/2), "e", strconv.Itoa(i/4)))
 		}
 	})
 }
@@ -75,7 +75,7 @@ func BenchmarkHeadStripeSeriesCreate_PreCreationFailure(b *testing.B) {
 	defer h.Close()
 
 	for i := 0; i < b.N; i++ {
-		h.getOrCreate(uint64(i), labels.FromStrings("a", strconv.Itoa(i)))
+		h.getOrCreate(uint64(i), labels.FromStrings(labels.MetricName, "test", "a", strconv.Itoa(i), "b", strconv.Itoa(i%10), "c", strconv.Itoa(i%100), "d", strconv.Itoa(i/2), "e", strconv.Itoa(i/4)))
 	}
 }
 
diff --git a/tsdb/index/postings.go b/tsdb/index/postings.go
index e909a3717..7bc5629ac 100644
--- a/tsdb/index/postings.go
+++ b/tsdb/index/postings.go
@@ -392,13 +392,14 @@ func (p *MemPostings) Add(id storage.SeriesRef, lset labels.Labels) {
 	p.mtx.Unlock()
 }
 
-func appendWithExponentialGrowth[T any](a []T, v T) []T {
+func appendWithExponentialGrowth[T any](a []T, v T) (_ []T, copied bool) {
 	if cap(a) < len(a)+1 {
 		newList := make([]T, len(a), len(a)*2+1)
 		copy(newList, a)
 		a = newList
+		copied = true
 	}
-	return append(a, v)
+	return append(a, v), copied
 }
 
 func (p *MemPostings) addFor(id storage.SeriesRef, l labels.Label) {
@@ -407,16 +408,26 @@ func (p *MemPostings) addFor(id storage.SeriesRef, l labels.Label) {
 		nm = map[string][]storage.SeriesRef{}
 		p.m[l.Name] = nm
 	}
-	list := appendWithExponentialGrowth(nm[l.Value], id)
+	list, copied := appendWithExponentialGrowth(nm[l.Value], id)
 	nm[l.Value] = list
 
-	if !p.ordered {
+	// Return if it shouldn't be ordered, if it only has one element or if it's already ordered.
+	// The invariant is that the first n-1 items in the list are already sorted.
+	if !p.ordered || len(list) == 1 || list[len(list)-1] >= list[len(list)-2] {
 		return
 	}
-	// There is no guarantee that no higher ID was inserted before as they may
-	// be generated independently before adding them to postings.
-	// We repair order violations on insert. The invariant is that the first n-1
-	// items in the list are already sorted.
+
+	if !copied {
+		// We have appended to the existing slice,
+		// and readers may already have a copy of this postings slice,
+		// so we need to copy it before sorting.
+		old := list
+		list = make([]storage.SeriesRef, len(old), cap(old))
+		copy(list, old)
+		nm[l.Value] = list
+	}
+
+	// Repair order violations.
 	for i := len(list) - 1; i >= 1; i-- {
 		if list[i] >= list[i-1] {
 			break
diff --git a/tsdb/index/postings_test.go b/tsdb/index/postings_test.go
index b41fb54e6..8ee9b9943 100644
--- a/tsdb/index/postings_test.go
+++ b/tsdb/index/postings_test.go
@@ -1507,3 +1507,58 @@ func TestMemPostings_PostingsForLabelMatchingHonorsContextCancel(t *testing.T) {
 	require.Error(t, p.Err())
 	require.Equal(t, failAfter+1, ctx.Count()) // Plus one for the Err() call that puts the error in the result.
 }
+
+func TestMemPostings_Unordered_Add_Get(t *testing.T) {
+	mp := NewMemPostings()
+	for ref := storage.SeriesRef(1); ref < 8; ref += 2 {
+		// First, add next series.
+		next := ref + 1
+		mp.Add(next, labels.FromStrings(labels.MetricName, "test", "series", strconv.Itoa(int(next))))
+		nextPostings := mp.Get(labels.MetricName, "test")
+
+		// Now add current ref.
+		mp.Add(ref, labels.FromStrings(labels.MetricName, "test", "series", strconv.Itoa(int(ref))))
+
+		// Next postings should still reference the next series.
+		nextExpanded, err := ExpandPostings(nextPostings)
+		require.NoError(t, err)
+		require.Len(t, nextExpanded, int(ref))
+		require.Equal(t, next, nextExpanded[len(nextExpanded)-1])
+	}
+}
+
+func TestMemPostings_Concurrent_Add_Get(t *testing.T) {
+	refs := make(chan storage.SeriesRef)
+	wg := sync.WaitGroup{}
+	wg.Add(1)
+	t.Cleanup(wg.Wait)
+	t.Cleanup(func() { close(refs) })
+
+	mp := NewMemPostings()
+	go func() {
+		defer wg.Done()
+		for ref := range refs {
+			mp.Add(ref, labels.FromStrings(labels.MetricName, "test", "series", strconv.Itoa(int(ref))))
+			p := mp.Get(labels.MetricName, "test")
+
+			_, err := ExpandPostings(p)
+			if err != nil {
+				t.Errorf("unexpected error: %s", err)
+			}
+		}
+	}()
+
+	for ref := storage.SeriesRef(1); ref < 8; ref += 2 {
+		// Add next ref in another goroutine so they would race.
+		refs <- ref + 1
+		// Add current ref here
+		mp.Add(ref, labels.FromStrings(labels.MetricName, "test", "series", strconv.Itoa(int(ref))))
+
+		// We don't read the value of the postings here,
+		// this is tested in TestMemPostings_Unordered_Add_Get where it's easier to achieve the determinism.
+		// This test just checks that there's no data race.
+		p := mp.Get(labels.MetricName, "test")
+		_, err := ExpandPostings(p)
+		require.NoError(t, err)
+	}
+}