diff --git a/engine/engine.go b/engine/engine.go
index 0108898d9..a7b4a0c25 100644
--- a/engine/engine.go
+++ b/engine/engine.go
@@ -571,8 +571,8 @@ func (q *compatibilityQuery) Exec(ctx context.Context) (ret *promql.Result) {
 	if err != nil {
 		return newErrResult(ret, err)
 	}
-	if extlabels.ContainsDuplicateLabelSet(resultSeries) {
-		return newErrResult(ret, extlabels.ErrDuplicateLabelSet)
+	if err := extlabels.CheckContainsDuplicateLabelSet(resultSeries); err != nil {
+		return newErrResult(ret, err)
 	}
 
 	series := make([]promql.Series, len(resultSeries))
diff --git a/engine/engine_test.go b/engine/engine_test.go
index 549e1c5d4..a65fba159 100644
--- a/engine/engine_test.go
+++ b/engine/engine_test.go
@@ -3759,7 +3759,7 @@ func TestInstantQuery(t *testing.T) {
 									t.Log("Applying comparison with NaN equality.")
 									equalsWithNaNs(t, oldResult, newResult)
 								} else if oldResult.Err != nil {
-									testutil.Equals(t, oldResult.Err.Error(), newResult.Err.Error())
+									testutil.NotOk(t, newResult.Err)
 								} else {
 									testutil.Equals(t, oldResult, newResult)
 								}
diff --git a/execution/function/histogram.go b/execution/function/histogram.go
index fa5292e12..b04706e08 100644
--- a/execution/function/histogram.go
+++ b/execution/function/histogram.go
@@ -183,8 +183,8 @@ func (o *histogramOperator) loadSeries(ctx context.Context) error {
 	if err != nil {
 		return err
 	}
-	if extlabels.ContainsDuplicateLabelSetAfterDroppingName(series) {
-		return extlabels.ErrDuplicateLabelSet
+	if err := extlabels.CheckContainsDuplicateLabelSetAfterDroppingName(series); err != nil {
+		return err
 	}
 
 	var (
diff --git a/extlabels/labels.go b/extlabels/labels.go
index f8be0c1fb..cab3a5e1d 100644
--- a/extlabels/labels.go
+++ b/extlabels/labels.go
@@ -4,16 +4,13 @@
 package extlabels
 
 import (
+	"fmt"
+
 	"github.com/cespare/xxhash/v2"
-	"github.com/efficientgo/core/errors"
 	"github.com/prometheus/prometheus/model/labels"
 )
 
-var (
-	ErrDuplicateLabelSet = errors.New("vector cannot contain metrics with the same labelset")
-)
-
-func ContainsDuplicateLabelSet(series []labels.Labels) bool {
+func CheckContainsDuplicateLabelSet(series []labels.Labels) error {
 	var (
 		buf  = make([]byte, 0, 256)
 		seen = make(map[uint64]struct{}, len(series))
@@ -25,14 +22,14 @@ func ContainsDuplicateLabelSet(series []labels.Labels) bool {
 		buf = s.Bytes(buf)
 		h := xxhash.Sum64(s.Bytes(buf))
 		if _, ok := seen[h]; ok {
-			return true
+			return fmt.Errorf("vector cannot contain metrics with the same labelset (%s)", s.String())
 		}
 		seen[h] = struct{}{}
 	}
-	return false
+	return nil
 }
 
-func ContainsDuplicateLabelSetAfterDroppingName(series []labels.Labels) bool {
+func CheckContainsDuplicateLabelSetAfterDroppingName(series []labels.Labels) error {
 	var (
 		buf  = make([]byte, 0, 256)
 		seen = make(map[uint64]struct{}, len(series))
@@ -48,11 +45,11 @@ func ContainsDuplicateLabelSetAfterDroppingName(series []labels.Labels) bool {
 
 		h := xxhash.Sum64(lbls.Bytes(buf))
 		if _, ok := seen[h]; ok {
-			return true
+			return fmt.Errorf("vector cannot contain metrics with same labelset (%s)", s.String())
 		}
 		seen[h] = struct{}{}
 	}
-	return false
+	return nil
 }
 
 // DropMetricName removes the __name__ label and returns the dropped name and remaining labels.