**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

airflow/operators/python.py
'prev_start_date_success',
    }
    AIRFLOW_SERIALIZABLE_CONTEXT_KEYS = {'macros', 'conf', 'dag', 'dag_run', 'task'}
    @apply_defaults
    def __init__(  
        self,
        *,
        python_callable: Callable,
        requirements: Optional[Iterable[str]] = None,
        python_version: Optional[Union[str, int, float]] = None,
        use_dill: bool = False,
        system_site_packages: bool = True,
        op_args: Optional[List] = None,
        op_kwargs: Optional[Dict] = None,
        string_args: Optional[Iterable[str]] = None,
        templates_dict: Optional[Dict] = None,
        templates_exts: Optional[List[str]] = None,
        **kwargs,
    ):
        if (
            not isinstance(python_callable, types.FunctionType)
            or isinstance(python_callable, types.LambdaType)
            and python_callable.__name__ == "<lambda>"
        ):
            raise AirflowException('PythonVirtualenvOperator only supports functions for python_callable arg')
        if (
            python_version
            and str(python_version)[0] != str(sys.version_info.major)
            and (op_args or op_kwargs)
        ):