How do I authorise more vehicles for the same user?

[anentropic]

The docs for getAuthUrl say:

https://github.com/smartcar/node-sdk/blob/master/doc/readme.md#authclientgetauthurlscope-options--string

By default users are not shown the permission dialog if they have already approved the set of scopes for this application.

But I can't see how to reach this behaviour. Working with the demo app https://github.com/smartcar/getting-started-express in test mode what I see is that the permission dialog is always displayed, and upon completion of the flow the backend receives a fresh access token but if I call getUser with it then it resolves to the same user id. But whichever vehicles are authorised replace any previously authorised vehicles for that user.

Is this a peculiarity of the test environment, or that's how it works in live too?

Is there a way to authorise additional vehicles for an existing user id?