**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
def f():
            {}.iteritems()  
            return True
        self._run_as_operator(f, python_version='2.7', requirements=['dill'])
    def test_python_3(self):
        def f():
            import sys  
            print(sys.version)
            try:
                {}.iteritems()  
            except AttributeError:
                return
            raise Exception
        self._run_as_operator(f, python_version=3, use_dill=False, requirements=['dill'])
    @staticmethod
    def _invert_python_major_version():
        if sys.version_info[0] == 2:
            return 3
        else:
            return 2
    def test_wrong_python_op_args(self):
        if sys.version_info[0] == 2:
            version = 3
        else:
            version = 2
        def f():
            pass
        with self.assertRaises(AirflowException):
            self._run_as_operator(f, python_version=version, op_args=[1])
    def test_without_dill(self):