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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
if head:
                    continue
                if mod_name == '__init__':
                    self.log.warning("Found __init__.%s at root of %s", ext, filepath)
                self.log.debug("Reading %s from %s", zip_info.filename, filepath)
                if not might_contain_dag(zip_info.filename, safe_mode, current_zip_file):
                    
                    
                    if not self.has_logged:
                        self.has_logged = True
                        self.log.info(
                            "File %s:%s assumed to contain no DAGs. Skipping.", filepath, zip_info.filename
                        )
                    continue
                if mod_name in sys.modules:
                    del sys.modules[mod_name]
                try:
                    sys.path.insert(0, filepath)
                    current_module = importlib.import_module(mod_name)
                    mods.append(current_module)
                except Exception as e:  
                    self.log.exception("Failed to import: %s", filepath)
                    if self.dagbag_import_error_tracebacks:
                        self.import_errors[filepath] = traceback.format_exc(
                            limit=-self.dagbag_import_error_traceback_depth
                        )
                    else:
                        self.import_errors[filepath] = str(e)
        return mods
    def _process_modules(self, filepath, mods, file_last_changed_on_disk):