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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/models/dagbag.py
"The store_serialized_dags parameter has been deprecated. "
                "You should pass the read_dags_from_db parameter.",
                DeprecationWarning,
                stacklevel=2,
            )
            read_dags_from_db = store_serialized_dags
        dag_folder = dag_folder or settings.DAGS_FOLDER
        self.dag_folder = dag_folder
        self.dags: Dict[str, DAG] = {}
        
        self.file_last_changed: Dict[str, datetime] = {}
        self.import_errors: Dict[str, str] = {}
        self.has_logged = False
        self.read_dags_from_db = read_dags_from_db
        
        self.dags_last_fetched: Dict[str, datetime] = {}
        
        self.dags_hash: Dict[str, str] = {}
        self.dagbag_import_error_tracebacks = conf.getboolean('core', 'dagbag_import_error_tracebacks')
        self.dagbag_import_error_traceback_depth = conf.getint('core', 'dagbag_import_error_traceback_depth')
        self.collect_dags(
            dag_folder=dag_folder,
            include_examples=include_examples,
            include_smart_sensor=include_smart_sensor,
            safe_mode=safe_mode,
        )
        
        
        self.load_op_links = load_op_links
    def size(self) -> int: