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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
"""Redirect to home if TI does not exist or if log handler is local"""
        url_template = "redirect_to_external_log?dag_id={}&task_id={}&execution_date={}&try_number={}"
        try_number = 1
        url = url_template.format(self.DAG_ID, task_id, quote_plus(self.DEFAULT_DATE.isoformat()), try_number)
        response = self.client.get(url)
        assert 302 == response.status_code
        assert 'http://localhost/home' == response.headers['Location']
    @mock.patch('airflow.utils.log.log_reader.TaskLogReader.log_handler', new_callable=PropertyMock)
    def test_redirect_to_external_log_with_external_log_handler(self, mock_log_handler):
        class ExternalHandler(ExternalLoggingMixin):
            EXTERNAL_URL = 'http://external-service.com'
            def get_external_log_url(self, *args, **kwargs):
                return self.EXTERNAL_URL
        mock_log_handler.return_value = ExternalHandler()
        url_template = "redirect_to_external_log?dag_id={}&task_id={}&execution_date={}&try_number={}"
        try_number = 1
        url = url_template.format(
            self.DAG_ID, self.TASK_ID, quote_plus(self.DEFAULT_DATE.isoformat()), try_number
        )
        response = self.client.get(url)
        assert 302 == response.status_code
        assert ExternalHandler.EXTERNAL_URL == response.headers['Location']
class ViewWithDateTimeAndNumRunsAndDagRunsFormTester: