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**Description**

When using decorators to define dags, e.g. dag_1.py:

```python
from airflow.decorators import dag, task
from airflow.utils.dates import days_ago

DEFAULT_ARGS = {
    "owner": "airflow",
}


@task
def some_task():
    pass


@dag(
    default_args=DEFAULT_ARGS,
    schedule_interval=None,
    start_date=days_ago(2),
)
def my_dag():
    some_task()


DAG_1 = my_dag()

```

and

dag_2.py:

```python
from airflow.decorators import dag, task
from airflow.utils.dates import days_ago

DEFAULT_ARGS = {
    "owner": "airflow",
}


@task
def some_other_task():
    pass


@dag(
    default_args=DEFAULT_ARGS,
    schedule_interval=None,
    start_date=days_ago(2),
)
def my_dag():
    some_other_task()


DAG_2 = my_dag()

```

We have two different dags which have been written in isolation, but by sheer bad luck both define `my_dag()`. This seems fine for each file in isolation, but on the airflow UI, we only end up seeing one entry for `my_dag`, where it has picked up `dag_1.py` and ignored `dag_2.py`.

**Use case / motivation**

We currently end up with only one DAG showing up on the UI, and no indication as to why the other one hasn't appeared.

Suggestion: popup similar to 'DAG import error' to highlight what needs changing in one of the DAG files in order for both to show up ("DAG import error: duplicate dag names found - please review {duplicate files} and ensure all dag definitions are unique"?)

**Are you willing to submit a PR?**

No time to spare on this at present

**Related Issues**

I haven't found any related issues with the search function.
tests/core/test_impersonation_tests.py
check_original_docker_image()
        grant_permissions()
        add_default_pool_if_not_exists()
        self.dagbag = models.DagBag(
            dag_folder=TEST_DAG_CORRUPTED_FOLDER,
            include_examples=False,
        )
        logger.info('Loaded DAGS:')
        logger.info(self.dagbag.dagbag_report())
        create_user()
    def tearDown(self):
        subprocess.check_output(['sudo', 'userdel', '-r', TEST_USER])
        revoke_permissions()
    def run_backfill(self, dag_id, task_id):
        dag = self.dagbag.get_dag(dag_id)
        dag.clear()
        BackfillJob(dag=dag, start_date=DEFAULT_DATE, end_date=DEFAULT_DATE).run()
        ti = models.TaskInstance(task=dag.get_task(task_id), execution_date=DEFAULT_DATE)
        ti.refresh_from_db()
        assert ti.state == State.SUCCESS
    @mock_custom_module_path(TEST_UTILS_FOLDER)
    def test_impersonation_custom(self):
        """
        Tests that impersonation using a unix user works with custom packages in
        PYTHONPATH
        """
        
        assert 'PYTHONPATH' in os.environ
        self.run_backfill('impersonation_with_custom_pkg', 'exec_python_fn')