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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
DAG_ID = 'dag_for_testing_dt_nr_dr_form'
    DEFAULT_DATE = datetime(2017, 9, 1)
    RUNS_DATA = [
        ('dag_run_for_testing_dt_nr_dr_form_4', datetime(2018, 4, 4)),
        ('dag_run_for_testing_dt_nr_dr_form_3', datetime(2018, 3, 3)),
        ('dag_run_for_testing_dt_nr_dr_form_2', datetime(2018, 2, 2)),
        ('dag_run_for_testing_dt_nr_dr_form_1', datetime(2018, 1, 1)),
    ]
    def __init__(self, test, endpoint):
        self.test = test
        self.endpoint = endpoint
        self.runs = []
    def setup(self):
        dagbag = self.test.app.dag_bag
        dag = DAG(self.DAG_ID, start_date=self.DEFAULT_DATE)
        dagbag.bag_dag(dag=dag, root_dag=dag)
        for run_data in self.RUNS_DATA:
            run = dag.create_dagrun(
                run_id=run_data[0], execution_date=run_data[1], state=State.SUCCESS, external_trigger=True
            )
            self.runs.append(run)
    def teardown(self):
        self.test.session.query(DagRun).filter(DagRun.dag_id == self.DAG_ID).delete()
        self.test.session.commit()
        self.test.session.close()
    def assert_base_date_and_num_runs(self, base_date, num_runs, data):