title: How to authenticate the same user on multiple clients using OAuth2?
body: <p>I'm using the Resource Owner Password Credentials flow to authenticate my iOS app against an API that I provide. This is working fine, and I get the access token and can refresh tokens when they expire.</p>
  
  <p>However if a user installs the app on multiple clients e.g. iPhone and iPad and authenticates with the same username and password they both get the same access token. The client that is first to refresh the access token works fine using the new access token, but the other client no longer works as it is using the previous access token.</p>
  
  <p>The obvious solution is to allow a user (resource owner) to have multiple access tokens and return a new access token every time they authenticate. This then presents the problem of how should I clean up the old access tokens?</p>
  
  <p>Or is there a better way of authenticating the same user on multiple clients?</p>