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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.
airflow/exceptions.py
"""
    Base class for all Airflow's errors.
    Each custom exception should be derived from this class
    """
    status_code = 500
class AirflowBadRequest(AirflowException):
    """Raise when the application or server cannot handle the request"""
    status_code = 400
class AirflowNotFoundException(AirflowException):
    """Raise when the requested object/resource is not available in the system"""
    status_code = 404
class AirflowConfigException(AirflowException):
    """Raise when there is configuration problem"""
class AirflowSensorTimeout(AirflowException):
    """Raise when there is a timeout on sensor polling"""
class AirflowRescheduleException(AirflowException):
    """
    Raise when the task should be re-scheduled at a later time.
    :param reschedule_date: The date when the task should be rescheduled
    :type reschedule_date: datetime.datetime
    """
    def __init__(self, reschedule_date):
        super().__init__()
        self.reschedule_date = reschedule_date
class AirflowSmartSensorException(AirflowException):