[BUG] Parallel Gateway with Multi-Instance Task Generates Duplicate Tasks When Moving Executions to External Activity

- Description
Parallel Gateway with Multi-Instance Task Generates Duplicate Tasks When Moving Executions to External Activity. Flowable v6.8.0 is ok.

- Version
Flowable Version: 7.1.0

- Unit Test

[test.bpmn20.zip](https://github.com/user-attachments/files/20201885/test.bpmn20.zip)

```java
@Test
@Deployment(resources = { "org/flowable/engine/test/api/test.bpmn20.xml" })
public void test() {
ProcessInstance processInstance = runtimeService.startProcessInstanceByKey("test");
Task task = taskService.createTaskQuery().processInstanceId(processInstance.getId()).singleResult();
assertThat(task.getTaskDefinitionKey()).isEqualTo("Starter");

// Complete task Starter
taskService.complete(task.getId());

// Current Tasks
List currentActivities = new ArrayList<>();
currentActivities.add("TaskA");
currentActivities.add("TaskB");

changeStateEventListener.clear();

runtimeService.createChangeActivityStateBuilder()
.processInstanceId(processInstance.getId())
.moveActivityIdsToSingleActivityId(currentActivities, "Starter")
.changeState();

List tasks = taskService.createTaskQuery().processInstanceId(processInstance.getId()).list();
assertThat(tasks).hasSize(1);
}
```