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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
with conf_vars({('logging', 'logging_config_class'): 'airflow_local_settings.LOGGING_CONFIG'}):
            self.appbuilder = self.app.appbuilder  
            self.app.config['WTF_CSRF_ENABLED'] = False
            self.client = self.app.test_client()
            settings.configure_orm()
            self.login()
            dagbag = self.app.dag_bag
            dag = DAG(self.DAG_ID, start_date=self.DEFAULT_DATE)
            dag_removed = DAG(self.DAG_ID_REMOVED, start_date=self.DEFAULT_DATE)
            dagbag.bag_dag(dag=dag, root_dag=dag)
            dagbag.bag_dag(dag=dag_removed, root_dag=dag_removed)
            
            with mock.patch.object(settings, "STORE_DAG_CODE", False):
                dag.sync_to_db()
                dag_removed.sync_to_db()
                dagbag.sync_to_db()
            with create_session() as session:
                self.ti = TaskInstance(
                    task=DummyOperator(task_id=self.TASK_ID, dag=dag), execution_date=self.DEFAULT_DATE
                )
                self.ti.try_number = 1
                self.ti_removed_dag = TaskInstance(
                    task=DummyOperator(task_id=self.TASK_ID, dag=dag_removed),
                    execution_date=self.DEFAULT_DATE,
                )
                self.ti_removed_dag.try_number = 1
                session.merge(self.ti)
                session.merge(self.ti_removed_dag)
    def tearDown(self):