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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/exceptions.py
"""Information about a single error in a file."""
    line_no: Optional[int]
    message: str
    def __str__(self):
        return f"{self.message}. Line number: s{str(self.line_no)},"
class AirflowFileParseException(AirflowException):
    """
    Raises when connection or variable file can not be parsed
    :param msg: The human-readable description of the exception
    :param file_path: A processed file that contains errors
    :param parse_errors: File syntax errors
    """
    def __init__(self, msg: str, file_path: str, parse_errors: List[FileSyntaxError]) -> None:
        super().__init__(msg)
        self.msg = msg
        self.file_path = file_path
        self.parse_errors = parse_errors
    def __str__(self):
        result = f"{self.msg}\nFilename: {self.file_path}\n\n"
        for error_no, parse_error in enumerate(self.parse_errors, 1):
            result += "=" * 20 + f" Parse error {error_no:3} " + "=" * 20 + "\n"
            result += f"{parse_error.message}\n"
            if parse_error.line_no:
                result += f"Line number:  {parse_error.line_no}\n"
                if parse_error.line_no and is_tty():
                    result += "\n" + prepare_code_snippet(self.file_path, parse_error.line_no) + "\n"
        return result
class ConnectionNotUnique(AirflowException):
    """Raise when multiple values are found for the same conn_id"""