<!--

Welcome to Apache Airflow!  For a smooth issue process, try to answer the following questions.
Don't worry if they're not all applicable; just try to include what you can :-)

If you need to include code snippets or logs, please put them in fenced code
blocks.  If they're super-long, please use the details tag like
<details><summary>super-long log</summary> lots of stuff </details>

Please delete these comment blocks before submitting the issue.

-->

**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/models/test_dagbag.py
#
import inspect
import os
import shutil
import textwrap
import unittest
from datetime import datetime, timezone
from tempfile import NamedTemporaryFile, mkdtemp
from unittest import mock
from unittest.mock import patch
from freezegun import freeze_time
from sqlalchemy import func
from sqlalchemy.exc import OperationalError
import airflow.example_dags
from airflow import models
from airflow.exceptions import SerializationError
from airflow.models import DagBag, DagModel
from airflow.models.serialized_dag import SerializedDagModel
from airflow.utils.dates import timezone as tz
from airflow.utils.session import create_session
from airflow.www.security import ApplessAirflowSecurityManager
from tests import cluster_policies
from tests.models import TEST_DAGS_FOLDER
from tests.test_utils import db
from tests.test_utils.asserts import assert_queries_count
from tests.test_utils.config import conf_vars
from tests.test_utils.permissions import delete_dag_specific_permissions
class TestDagBag(unittest.TestCase):
    @classmethod
    def setUpClass(cls):