diff --git a/promql/functions.go b/promql/functions.go
index 938eefdf00..d8b22ddd40 100644
--- a/promql/functions.go
+++ b/promql/functions.go
@@ -1689,6 +1689,9 @@ func (ev *evaluator) evalLabelJoin(ctx context.Context, args parser.Expressions)
 			matrix[i].DropName = el.DropName
 		}
 	}
+	if matrix.ContainsSameLabelset() {
+		ev.errorf("vector cannot contain metrics with the same labelset")
+	}
 
 	return matrix, ws
 }
diff --git a/promql/promqltest/testdata/functions.test b/promql/promqltest/testdata/functions.test
index 32838afe73..68cf5a09b4 100644
--- a/promql/promqltest/testdata/functions.test
+++ b/promql/promqltest/testdata/functions.test
@@ -499,6 +499,8 @@ eval instant at 20s timestamp(metric)
 load 5m
   testmetric{src="a",src1="b",src2="c",dst="original-destination-value"} 0
   testmetric{src="d",src1="e",src2="f",dst="original-destination-value"} 1
+  dup{label="a", this="a"} 1.0
+  dup{label="b", this="a"} 1.0
 
 # label_join joins all src values in order.
 eval instant at 0m label_join(testmetric, "dst", "-", "src", "src1", "src2")
@@ -530,6 +532,9 @@ eval instant at 0m label_join(testmetric1, "dst", ", ", "src", "src1", "src2")
   testmetric1{src="foo",src1="bar",src2="foobar",dst="foo, bar, foobar"} 0
   testmetric1{src="fizz",src1="buzz",src2="fizzbuzz",dst="fizz, buzz, fizzbuzz"} 1
 
+eval_fail instant at 0m label_join(dup, "label", "", "this")
+  expected_fail_message vector cannot contain metrics with the same labelset
+
 clear
 
 # Tests for vector.