**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
state=State.RUNNING,
        )
        task.run(start_date=DEFAULT_DATE, end_date=DEFAULT_DATE)
        self.assertEqual(1, len(recorded_calls))
        self._assert_calls_equal(
            recorded_calls[0],
            Call(
                an_int=4,
                a_date=date(2019, 1, 1),
                a_templated_string="dag {} ran on {}.".format(
                    self.dag.dag_id, DEFAULT_DATE.date().isoformat()
                ),
            ),
        )
    def test_python_operator_shallow_copy_attr(self):
        not_callable = lambda x: x
        original_task = PythonOperator(
            python_callable=not_callable,
            task_id='python_operator',
            op_kwargs={'certain_attrs': ''},
            dag=self.dag,
        )
        new_task = copy.deepcopy(original_task)
        
        self.assertEqual(
            id(original_task.op_kwargs['certain_attrs']), id(new_task.op_kwargs['certain_attrs'])
        )
        
        self.assertEqual(id(original_task.python_callable), id(new_task.python_callable))
    def test_conflicting_kwargs(self):