title: Apple TestFlight error 'Couldn't accept the invite'
body: <p>I have installed an app successfully as an internal tester. Now the app is ready for external testing, and has passed Apple review. In the email received from Apple, I clicked the <code>Open in TestFlight</code> button, which opened the TestFlight app and displayed the app details.</p>
  
  <p>Clicking the <code>INSTALL</code> button fails with the following alert:</p>
  
  <blockquote>
    <p><strong>Couldn't accept the invite</strong></p>
    
    <p>You've already accepted the invite for this app.</p>
    
    <p>OK</p>
  </blockquote>
  
  <p>The message does identify the problem. Any ideas how to resolve it?</p>