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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
last_name='dag_read_only',
                email='dag_read_only@fab.org',
                role=dag_read_only_role,
                password='dag_read_only',
            )
        
        all_dag_role = self.appbuilder.sm.add_role('all_dag_role')
        if username == 'all_dag_user' and not self.appbuilder.sm.find_user(username='all_dag_user'):
            self.appbuilder.sm.add_user(
                username='all_dag_user',
                first_name='all_dag_user',
                last_name='all_dag_user',
                email='all_dag_user@fab.org',
                role=all_dag_role,
                password='all_dag_user',
            )
        return super().login(username, password)
    def add_permission_for_role(self):
        self.logout()
        self.login(username='test', password='test')
        website_permission = self.appbuilder.sm.find_permission_view_menu(
            permissions.ACTION_CAN_READ, permissions.RESOURCE_WEBSITE
        )
        dag_tester_role = self.appbuilder.sm.find_role('dag_acl_tester')
        edit_perm_on_dag = self.appbuilder.sm.find_permission_view_menu(
            permissions.ACTION_CAN_EDIT, 'DAG:example_bash_operator'
        )
        self.appbuilder.sm.add_permission_role(dag_tester_role, edit_perm_on_dag)
        read_perm_on_dag = self.appbuilder.sm.find_permission_view_menu(
            permissions.ACTION_CAN_READ, 'DAG:example_bash_operator'