**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
ds_templated = DEFAULT_DATE.date().isoformat()
        self.assertEqual(1, len(recorded_calls))
        self._assert_calls_equal(
            recorded_calls[0],
            Call(
                4,
                date(2019, 1, 1),
                f"dag {self.dag.dag_id} ran on {ds_templated}.",
                Named(ds_templated, 'unchanged'),
            ),
        )
    def test_python_callable_keyword_arguments_are_templatized(self):
        """Test PythonOperator op_kwargs are templatized"""
        recorded_calls = []
        task = PythonOperator(
            task_id='python_operator',
            
            
            python_callable=build_recording_function(recorded_calls),
            op_kwargs={
                'an_int': 4,
                'a_date': date(2019, 1, 1),
                'a_templated_string': "dag {{dag.dag_id}} ran on {{ds}}.",
            },
            dag=self.dag,
        )
        self.dag.create_dagrun(
            run_type=DagRunType.MANUAL,
            execution_date=DEFAULT_DATE,
            start_date=DEFAULT_DATE,