text: Title: AssertionError: View function mapping is overwriting an existing endpoint function: api.specs
  Body: Hello,
  
  I'm working on an API using Flask (full library versions available below) and Flask-Restx and [importing via Blueprints](https://flask-restx.readthedocs.io/en/latest/scaling.html#use-with-blueprints) with a project structure like so:
  
  ```bash
  .
  ├── application
  │   ├── apis
  │   │   ├── __init__.py
  │   │   └── organisations.py
  │   ├── app.py
  │   └── config.py
  └── wsgi.py
  ```
  The code in `app.py` calls a blueprint created in `application/apis/__init__.py` as follows:
  
  **application/apis/__init__.py**
  ```python
  from flask import Blueprint
  from flask_restx import Api
  
  from .organisations import api as orgapi
  
  api_v1 = Blueprint('api', __name__)
  
  api = Api(
      api_v1,
      title='My API',
      version='1.0',
      description='Access to my api',
  )
  
  api.add_namespace(orgapi)
  ```
  **application/app.py**
  
  ```python
  # ...
  from application.apis import api
  
  # ...
  def create_app(config_name):
  
      app = Flask(__name__)
      # ...
      api.init_app(app)
      # Blueprints
      from application.apis import api_v1 
      app.register_blueprint(api_v1, url_prefix="/api/v1")
  ```
  The `organisations.py` code does not include any views at all, however when I try and access the application, I get the error in the title bar:
  
  **application/apis/organisations.py**
  ```python
  from flask_restx import Namespace, Resource, fields
  
  api = Namespace('organisation', description='Organisation related operations')
  ```
  
  The only reference I can find to this is a [stackoverflow](https://stackoverflow.com/questions/17256602/assertionerror-view-function-mapping-is-overwriting-an-existing-endpoint-functi) question, and [a github issue dating back to Flask 0.9 vs Flask 0.10](https://github.com/pallets/flask/issues/796), however given how old those questions are I'm pretty confident I'm just holding it wrong!
  
  #### Additional context
  
  **Library Versions (`pip freeze | grep -i flask`)**:
  ```
  Flask==1.1.2
  Flask-Admin==1.5.6
  Flask-Migrate==2.5.3
  flask-restx==0.2.0
  Flask-SQLAlchemy==2.4.4
  pytest-flask==1.0.0
  ```
  
  #### Full Stack Trace
  
  ```bash
  Traceback (most recent call last):
    File "/usr/local/lib/python3.9/site-packages/flask/_compat.py", line 39, in reraise
      raise value
    File "/opt/code/wsgi.py", line 5, in <module>
      app = create_app(os.environ["FLASK_CONFIG"])
    File "/opt/code/application/app.py", line 54, in create_app
      app.register_blueprint(api_v1, url_prefix="/api/v1")
    File "/usr/local/lib/python3.9/site-packages/flask/app.py", line 98, in wrapper_func
      return f(self, *args, **kwargs)
    File "/usr/local/lib/python3.9/site-packages/flask/app.py", line 1168, in register_blueprint
      blueprint.register(self, options, first_registration)
    File "/usr/local/lib/python3.9/site-packages/flask/blueprints.py", line 256, in register
      deferred(state)
    File "/usr/local/lib/python3.9/site-packages/flask/blueprints.py", line 294, in <lambda>
      self.record(lambda s: s.add_url_rule(rule, endpoint, view_func, **options))
    File "/usr/local/lib/python3.9/site-packages/flask_restx/api.py", line 809, in _blueprint_setup_add_url_rule_patch
      blueprint_setup.app.add_url_rule(
    File "/usr/local/lib/python3.9/site-packages/flask/app.py", line 98, in wrapper_func
      return f(self, *args, **kwargs)
    File "/usr/local/lib/python3.9/site-packages/flask/app.py", line 1282, in add_url_rule
      raise AssertionError(
  AssertionError: View function mapping is overwriting an existing endpoint function: api.specs
  ```