Please help resolve this issue.
Background:

1. User invited on their personal email to test an iOS app via Apple
   Testflight.(personal email is different from Apple Store/ Testflight
   email)
2. User takes the redeem / invitation code and uses it in
   another phone belonging to someone else User then asks for another
   invitation, to get the app on their own device, but it cannot be
   sent, because they have already accepted.
3. The user is then deleted
   and removed from any tester groups within Apple Testflight, and then
   re-invited afresh
4. User cannot use the new testflight redeem/
   invitation code . . . Keeps getting error “Couldn’t redeem
   TestFlight invitation Code …”…

Please advice how user can get past this issue, or what can I do to give them a working invitation?