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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
TI_ENDPOINT = '/taskreschedule/list/?_flt_0_execution_date={}'
    def test_start_date_filter(self):
        resp = self.client.get(self.TI_ENDPOINT.format(self.percent_encode('2018-10-09 22:44:31')))
        
        
        
        self.check_content_in_response('List Task Reschedule', resp)
class TestRenderedView(TestBase):
    def setUp(self):
        self.default_date = datetime(2020, 3, 1)
        self.dag = DAG(
            "testdag",
            start_date=self.default_date,
            user_defined_filters={"hello": lambda name: f'Hello {name}'},
            user_defined_macros={"fullname": lambda fname, lname: f'{fname} {lname}'},
        )
        self.task1 = BashOperator(task_id='task1', bash_command='{{ task_instance_key_str }}', dag=self.dag)
        self.task2 = BashOperator(
            task_id='task2', bash_command='echo {{ fullname("Apache", "Airflow") | hello }}', dag=self.dag
        )
        SerializedDagModel.write_dag(self.dag)
        with create_session() as session:
            session.query(RTIF).delete()
        self.app.dag_bag = mock.MagicMock(**{'get_dag.return_value': self.dag})
        super().setUp()
    def tearDown(self) -> None: