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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/api_connexion/endpoints/test_log_endpoint.py
self.log_dir = tmp_path
        dir_path = tmp_path / self.DAG_ID / self.TASK_ID / self.default_time.replace(':', '.')
        dir_path.mkdir(parents=True)
        log = dir_path / "1.log"
        log.write_text("Log for testing.")
        
        logging_config = copy.deepcopy(DEFAULT_LOGGING_CONFIG)
        logging_config['handlers']['task']['base_log_folder'] = self.log_dir
        logging_config['handlers']['task'][
            'filename_template'
        ] = '{{ ti.dag_id }}/{{ ti.task_id }}/{{ ts | replace(":", ".") }}/{{ try_number }}.log'
        logging.config.dictConfig(logging_config)
        yield
        logging.config.dictConfig(DEFAULT_LOGGING_CONFIG)
    def _prepare_db(self):
        dagbag = self.app.dag_bag  
        dag = DAG(self.DAG_ID, start_date=timezone.parse(self.default_time))
        dag.sync_to_db()
        dagbag.bag_dag(dag=dag, root_dag=dag)
        with create_session() as session:
            self.ti = TaskInstance(
                task=DummyOperator(task_id=self.TASK_ID, dag=dag),
                execution_date=timezone.parse(self.default_time),
            )
            self.ti.try_number = 1
            session.merge(self.ti)
    def teardown_method(self):