Why does uncaught exception get logged to Sentry twice?

I was testing how exceptions in my Flask app would get logged to Sentry and noticed that the same exception always gets logged twice, with sightly differing stack trace. Why does this happen and how should I prevent it?

For the first entry, I get the following stack trace:

  File "raven/middleware.py", line 35, in __call__
    iterable = self.application(environ, start_response)
  File "flask/app.py", line 1820, in wsgi_app
    response = self.make_response(self.handle_exception(e))
  File "flask/app.py", line 1403, in handle_exception
    reraise(exc_type, exc_value, tb)
  File "flask/app.py", line 1817, in wsgi_app
    response = self.full_dispatch_request()
  File "flask/app.py", line 1477, in full_dispatch_request
    rv = self.handle_user_exception(e)
  File "flask/app.py", line 1381, in handle_user_exception
    reraise(exc_type, exc_value, tb)
  File "flask/app.py", line 1475, in full_dispatch_request
    rv = self.dispatch_request()
  File "flask/app.py", line 1461, in dispatch_request
    return self.view_functions[rule.endpoint](**req.view_args)
  File "app/api_1_0/security/decorators.py", line 52, in decorated
    self._process(permission)
  File "app/api_1_0/security/decorators.py", line 27, in _process
    raise FooException('foo exception in _process')

For the second entry, the stack trace is as follows:

  File "flask/app.py", line 1817, in wsgi_app
    response = self.full_dispatch_request()
  File "flask/app.py", line 1477, in full_dispatch_request
    rv = self.handle_user_exception(e)
  File "flask/app.py", line 1381, in handle_user_exception
    reraise(exc_type, exc_value, tb)
  File "flask/app.py", line 1475, in full_dispatch_request
    rv = self.dispatch_request()
  File "flask/app.py", line 1461, in dispatch_request
    return self.view_functions[rule.endpoint](**req.view_args)
  File "app/api_1_0/security/decorators.py", line 52, in decorated
    self._process(permission)
  File "app/api_1_0/security/decorators.py", line 27, in _process
    raise FooException('foo exception in _process')

Solution

set flask debug to False

e.g.

app.run(debug=True)
to 
app.run(debug=False)

related issue on github: https://github.com/getsentry/raven-python/issues/412