diff --git a/promql/engine.go b/promql/engine.go
index 2051774af2..90fc78e8a0 100644
--- a/promql/engine.go
+++ b/promql/engine.go
@@ -720,6 +720,9 @@ func (ev *evaluator) rangeEval(f func([]Value, *EvalNodeHelper) Vector, exprs ..
 		// Make the function call.
 		enh.ts = ts
 		result := f(args, enh)
+		if result.ContainsSameLabelset() {
+			ev.errorf("vector cannot contain metrics with the same labelset")
+		}
 		enh.out = result[:0] // Reuse result vector.
 		// If this could be an instant query, shortcut so as not to change sort order.
 		if ev.endTimestamp == ev.startTimestamp {
@@ -883,6 +886,10 @@ func (ev *evaluator) eval(expr Expr) Value {
 				mat = append(mat, ss)
 			}
 		}
+		if mat.ContainsSameLabelset() {
+			ev.errorf("vector cannot contain metrics with the same labelset")
+		}
+
 		putPointSlice(points)
 		return mat
 
@@ -898,6 +905,9 @@ func (ev *evaluator) eval(expr Expr) Value {
 					mat[i].Points[j].V = -mat[i].Points[j].V
 				}
 			}
+			if mat.ContainsSameLabelset() {
+				ev.errorf("vector cannot contain metrics with the same labelset")
+			}
 		}
 		return mat
 
diff --git a/promql/functions.go b/promql/functions.go
index 46fa473974..21757da343 100644
--- a/promql/functions.go
+++ b/promql/functions.go
@@ -734,7 +734,6 @@ func funcLabelReplace(vals []Value, args Expressions, enh *EvalNodeHelper) Vecto
 		enh.dmn = make(map[uint64]labels.Labels, len(enh.out))
 	}
 
-	outSet := make(map[uint64]struct{}, len(vector))
 	for _, el := range vector {
 		h := el.Metric.Hash()
 		var outMetric labels.Labels
@@ -759,17 +758,10 @@ func funcLabelReplace(vals []Value, args Expressions, enh *EvalNodeHelper) Vecto
 			}
 		}
 
-		outHash := outMetric.Hash()
-		if _, ok := outSet[outHash]; ok {
-			panic(fmt.Errorf("duplicated label set in output of label_replace(): %s", el.Metric))
-		} else {
-			enh.out = append(enh.out,
-				Sample{
-					Metric: outMetric,
-					Point:  Point{V: el.Point.V},
-				})
-			outSet[outHash] = struct{}{}
-		}
+		enh.out = append(enh.out, Sample{
+			Metric: outMetric,
+			Point:  Point{V: el.Point.V},
+		})
 	}
 	return enh.out
 }
@@ -808,7 +800,6 @@ func funcLabelJoin(vals []Value, args Expressions, enh *EvalNodeHelper) Vector {
 		panic(fmt.Errorf("invalid destination label name in label_join(): %s", dst))
 	}
 
-	outSet := make(map[uint64]struct{}, len(vector))
 	srcVals := make([]string, len(srcLabels))
 	for _, el := range vector {
 		h := el.Metric.Hash()
@@ -833,17 +824,11 @@ func funcLabelJoin(vals []Value, args Expressions, enh *EvalNodeHelper) Vector {
 			outMetric = lb.Labels()
 			enh.dmn[h] = outMetric
 		}
-		outHash := outMetric.Hash()
 
-		if _, exists := outSet[outHash]; exists {
-			panic(fmt.Errorf("duplicated label set in output of label_join(): %s", el.Metric))
-		} else {
-			enh.out = append(enh.out, Sample{
-				Metric: outMetric,
-				Point:  Point{V: el.Point.V},
-			})
-			outSet[outHash] = struct{}{}
-		}
+		enh.out = append(enh.out, Sample{
+			Metric: outMetric,
+			Point:  Point{V: el.Point.V},
+		})
 	}
 	return enh.out
 }
diff --git a/promql/testdata/functions.test b/promql/testdata/functions.test
index 70baae5735..2ecad97889 100644
--- a/promql/testdata/functions.test
+++ b/promql/testdata/functions.test
@@ -514,3 +514,11 @@ eval instant at 0m days_in_month(vector(1454284800))
 eval instant at 0m days_in_month(vector(1485907200))
   {} 28
 
+clear
+
+# Test duplicate labelset in promql output.
+load 5m
+  testmetric1{src="a",dst="b"} 0
+  testmetric2{src="a",dst="b"} 1
+
+eval_fail instant at 0m changes({__name__=~'testmetric1|testmetric2'}[5m])
\ No newline at end of file
diff --git a/promql/testdata/legacy.test b/promql/testdata/legacy.test
index daa1e473e5..5ed4421334 100644
--- a/promql/testdata/legacy.test
+++ b/promql/testdata/legacy.test
@@ -357,3 +357,12 @@ load 1h
 eval instant at 0h testmetric
 	testmetric{aa="bb"} 1
 	testmetric{a="abb"} 2
+
+clear
+
+# Test duplicate labelset in promql output.
+load 5m
+  testmetric1{src="a",dst="b"} 0
+  testmetric2{src="a",dst="b"} 1
+
+eval_fail instant at 0m ceil({__name__=~'testmetric1|testmetric2'})
\ No newline at end of file
diff --git a/promql/testdata/operators.test b/promql/testdata/operators.test
index 7bac547801..067e86839f 100644
--- a/promql/testdata/operators.test
+++ b/promql/testdata/operators.test
@@ -367,3 +367,12 @@ eval instant at 5m metricA + ignoring() metricB
 
 eval instant at 5m metricA + metricB
   {baz="meh"} 7
+
+clear
+
+# Test duplicate labelset in promql output.
+load 5m
+  testmetric1{src="a",dst="b"} 0
+  testmetric2{src="a",dst="b"} 1
+
+eval_fail instant at 0m -{__name__=~'testmetric1|testmetric2'}
\ No newline at end of file
diff --git a/promql/value.go b/promql/value.go
index fe902cd23a..a577030815 100644
--- a/promql/value.go
+++ b/promql/value.go
@@ -140,6 +140,22 @@ func (vec Vector) String() string {
 	return strings.Join(entries, "\n")
 }
 
+// ContainsSameLabelset checks if a vector has samples with the same labelset
+// Such a behaviour is semantically undefined
+// https://github.com/prometheus/prometheus/issues/4562
+func (vec Vector) ContainsSameLabelset() bool {
+	l := make(map[uint64]struct{}, len(vec))
+	for _, s := range vec {
+		hash := s.Metric.Hash()
+		if _, ok := l[hash]; ok {
+			return true
+		} else {
+			l[hash] = struct{}{}
+		}
+	}
+	return false
+}
+
 // Matrix is a slice of Seriess that implements sort.Interface and
 // has a String method.
 type Matrix []Series
@@ -159,6 +175,22 @@ func (m Matrix) Len() int           { return len(m) }
 func (m Matrix) Less(i, j int) bool { return labels.Compare(m[i].Metric, m[j].Metric) < 0 }
 func (m Matrix) Swap(i, j int)      { m[i], m[j] = m[j], m[i] }
 
+// ContainsSameLabelset checks if a matrix has samples with the same labelset
+// Such a behaviour is semantically undefined
+// https://github.com/prometheus/prometheus/issues/4562
+func (m Matrix) ContainsSameLabelset() bool {
+	l := make(map[uint64]struct{}, len(m))
+	for _, ss := range m {
+		hash := ss.Metric.Hash()
+		if _, ok := l[hash]; ok {
+			return true
+		} else {
+			l[hash] = struct{}{}
+		}
+	}
+	return false
+}
+
 // Result holds the resulting value of an execution or an error
 // if any occurred.
 type Result struct {