**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
from airflow.models import DAG, DagRun, TaskInstance as TI
from airflow.models.baseoperator import BaseOperator
from airflow.models.taskinstance import clear_task_instances, set_current_context
from airflow.models.xcom_arg import XComArg
from airflow.operators.dummy_operator import DummyOperator
from airflow.operators.python import (
    BranchPythonOperator,
    PythonOperator,
    PythonVirtualenvOperator,
    ShortCircuitOperator,
    get_current_context,
    task as task_decorator,
)
from airflow.utils import timezone
from airflow.utils.dates import days_ago
from airflow.utils.session import create_session
from airflow.utils.state import State
from airflow.utils.types import DagRunType
from tests.test_utils.db import clear_db_runs
DEFAULT_DATE = timezone.datetime(2016, 1, 1)
END_DATE = timezone.datetime(2016, 1, 2)
INTERVAL = timedelta(hours=12)
FROZEN_NOW = timezone.datetime(2016, 1, 2, 12, 1, 1)
TI_CONTEXT_ENV_VARS = [
    'AIRFLOW_CTX_DAG_ID',
    'AIRFLOW_CTX_TASK_ID',
    'AIRFLOW_CTX_EXECUTION_DATE',
    'AIRFLOW_CTX_DAG_RUN_ID',
]
class Call: