**Apache Airflow version**:
2.0 / master

**Environment**:

breeze

**What happened**:

Using this DAG:

```py
from airflow.operators.bash import BashOperator
from airflow.operators.python import task
from airflow.models import DAG
from airflow.utils.task_group import TaskGroup

@task
def show():
    print("Cats are awesome!")

with DAG(
    "using_task_group",
    default_args={'owner': 'airflow'},
    start_date=days_ago(2),
    schedule_interval=None,
) as dag3:
    start_task = BashOperator(
        task_id="start_task",
        bash_command="echo start",
    )

    end_task = BashOperator(
        task_id="end_task",
        bash_command="echo end",
    )

    with TaskGroup(group_id="show_tasks") as tg1:
        previous_show = show()
        for _ in range(100):
            next_show = show()
            previous_show >> next_show
            previous_show = next_show
```
I get:
```
Broken DAG: [/files/dags/test.py] Traceback (most recent call last):
  File "/opt/airflow/airflow/models/baseoperator.py", line 410, in __init__
    task_group.add(self)
  File "/opt/airflow/airflow/utils/task_group.py", line 140, in add
    raise DuplicateTaskIdFound(f"Task id '{key}' has already been added to the DAG")
airflow.exceptions.DuplicateTaskIdFound: Task id 'show_tasks.show' has already been added to the DAG
```

If I remove the task group the task are generated as expected.

**What you expected to happen**:

I expect to be able to generate tasks dynamically using TaskGroup and task decoratos.

**How to reproduce it**:

Use the DAG from above.

**Anything else we need to know**:

N/A

tests/operators/test_python.py
[
        ("task1", [State.SUCCESS, State.SUCCESS, State.SUCCESS]),
        ("join", [State.SUCCESS, State.SKIPPED, State.SUCCESS]),
    ],
)
def test_empty_branch(choice, expected_states):
    """
    Tests that BranchPythonOperator handles empty branches properly.
    """
    with DAG(
        'test_empty_branch',
        start_date=DEFAULT_DATE,
    ) as dag:
        branch = BranchPythonOperator(task_id='branch', python_callable=lambda: choice)
        task1 = DummyOperator(task_id='task1')
        join = DummyOperator(task_id='join', trigger_rule="none_failed_or_skipped")
        branch >> [task1, join]
        task1 >> join
    dag.clear(start_date=DEFAULT_DATE)
    task_ids = ["branch", "task1", "join"]
    tis = {}
    for task_id in task_ids:
        task_instance = TI(dag.get_task(task_id), execution_date=DEFAULT_DATE)
        tis[task_id] = task_instance
        task_instance.run()
    def get_state(ti):
        ti.refresh_from_db()
        return ti.state
    assert [get_state(tis[task_id]) for task_id in task_ids] == expected_states