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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
self.log.debug('Loaded DAG %s', dag)
        except AirflowDagCycleException as cycle_exception:
            
            self.log.exception('Exception bagging dag: %s', dag.dag_id)
            
            
            if dag == root_dag:
                for subdag in subdags:
                    if subdag.dag_id in self.dags:
                        del self.dags[subdag.dag_id]
            raise cycle_exception
    def collect_dags(
        self,
        dag_folder: Union[str, "pathlib.Path", None] = None,
        only_if_updated: bool = True,
        include_examples: bool = conf.getboolean('core', 'LOAD_EXAMPLES'),
        include_smart_sensor: bool = conf.getboolean('smart_sensor', 'USE_SMART_SENSOR'),
        safe_mode: bool = conf.getboolean('core', 'DAG_DISCOVERY_SAFE_MODE'),
    ):
        """
        Given a file path or a folder, this method looks for python modules,
        imports them and adds them to the dagbag collection.
        Note that if a ``.airflowignore`` file is found while processing
        the directory, it will behave much like a ``.gitignore``,
        ignoring files that match any of the regex patterns specified
        in the file.
        **Note**: The patterns in .airflowignore are treated as
        un-anchored regexes, not shell-like glob patterns.
        """
        if self.read_dags_from_db: