# UNDF: UNDF-2026-000000761
# UNDF: (leave blank)
--- a/flink-table/flink-table-planner/src/main/java/org/apache/flink/table/planner/plan/nodes/exec/processor/MultipleInputNodeCreationProcessor.java
+++ b/flink-table/flink-table-planner/src/main/java/org/apache/flink/table/planner/plan/nodes/exec/processor/MultipleInputNodeCreationProcessor.java
@@ -534,7 +534,7 @@ public class MultipleInputNodeCreationProcessor implements DAGProcessor {
         // calculate the inputs of the multiple input node
         List<Tuple3<ExecNode<?>, InputProperty, ExecEdge>> inputs = new ArrayList<>();
+        Set<ExecNodeWrapper> membersSet = new HashSet<>(group.members);
         for (ExecNodeWrapper member : group.members) {
             for (int i = 0; i < member.inputs.size(); i++) {
                 ExecNodeWrapper memberInput = member.inputs.get(i);
-                if (group.members.contains(memberInput)) {
+                if (membersSet.contains(memberInput)) {
                     continue;
                 }

@@ -705,10 +705,12 @@ public class MultipleInputNodeCreationProcessor implements DAGProcessor {
             Preconditions.checkNotNull(
                     root, "Multiple input group does not have a root. This is a bug.");
-            Set<ExecNodeWrapper> sameGroupInputWrappers = new HashSet<>();
+            Set<ExecNodeWrapper> membersSet = new HashSet<>(members);
+            Set<ExecNodeWrapper> sameGroupInputWrappers = new HashSet<>();
             for (ExecNodeWrapper inputWrapper : root.inputs) {
-                if (members.contains(inputWrapper)) {
+                if (membersSet.contains(inputWrapper)) {
                     sameGroupInputWrappers.add(inputWrapper);
                 }
             }