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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/www/test_views.py
url_template = (
            "get_logs_with_metadata?dag_id={}&task_id={}&execution_date={}&try_number={}&metadata=null"
        )
        response = self.client.get(
            url_template.format(self.DAG_ID, self.TASK_ID, quote_plus(self.DEFAULT_DATE.isoformat()), 1),
            data=dict(username='test', password='test'),
            follow_redirects=True,
        )
        assert '"message":' in response.data.decode('utf-8')
        assert '"metadata":' in response.data.decode('utf-8')
        assert 'Log for testing.' in response.data.decode('utf-8')
        assert 200 == response.status_code
    @mock.patch("airflow.utils.log.file_task_handler.FileTaskHandler.read")
    def test_get_logs_with_metadata_for_removed_dag(self, mock_read):
        mock_read.return_value = (['airflow log line'], [{'end_of_log': True}])
        url_template = (
            "get_logs_with_metadata?dag_id={}&task_id={}&execution_date={}&try_number={}&metadata={}"
        )
        url = url_template.format(
            self.DAG_ID_REMOVED, self.TASK_ID, quote_plus(self.DEFAULT_DATE.isoformat()), 1, json.dumps({})
        )
        response = self.client.get(url, data=dict(username='test', password='test'), follow_redirects=True)
        assert '"message":' in response.data.decode('utf-8')
        assert '"metadata":' in response.data.decode('utf-8')
        assert 'airflow log line' in response.data.decode('utf-8')
        assert 200 == response.status_code
    def test_get_logs_response_with_ti_equal_to_none(self):