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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/models/test_dagbag.py
"""With safe mode disabled, an empty python file should be discovered."""
        with NamedTemporaryFile(dir=self.empty_dir, suffix=".py") as f:
            with conf_vars({('core', 'dags_folder'): self.empty_dir}):
                dagbag = models.DagBag(include_examples=False, safe_mode=False)
            assert len(dagbag.dagbag_stats) == 1
            assert dagbag.dagbag_stats[0].file == f"/{os.path.basename(f.name)}"
    def test_process_file_that_contains_multi_bytes_char(self):
        """
        test that we're able to parse file that contains multi-byte char
        """
        with NamedTemporaryFile() as f:
            f.write('\u3042'.encode())  
            f.flush()
            dagbag = models.DagBag(dag_folder=self.empty_dir, include_examples=False)
            assert [] == dagbag.process_file(f.name)
    def test_zip_skip_log(self):
        """
        test the loading of a DAG from within a zip file that skips another file because
        it doesn't have "airflow" and "DAG"
        """
        with self.assertLogs() as cm:
            test_zip_path = os.path.join(TEST_DAGS_FOLDER, "test_zip.zip")
            dagbag = models.DagBag(dag_folder=test_zip_path, include_examples=False)
            assert dagbag.has_logged
            assert (
                f'INFO:airflow.models.dagbag.DagBag:File {test_zip_path}:file_no_airflow_dag.py '
                'assumed to contain no DAGs. Skipping.' in cm.output
            )
    def test_zip(self):