TestFlight “The invitation has already been redeemed” - what to do?

In TestFlight, encountering the error message "The invitation has already been redeemed" can be frustrating. If you come across this issue, the recommended course of action is to request a new invite from the developer. Here are some possible solutions to this problem:

1. TestFlight restricts external testing to one device per external tester. If you are attempting to redeem an invite on multiple devices, this could be the cause of the error message.
2. As a developer, you can try removing the tester from both internal and external tester lists and then re-adding them as an internal tester. Subsequently, generate a new TestFlight redeem code for the tester to use.
3. When inviting external testers, keep in mind that each external tester is limited to one device. If multiple individuals are using the same iTunes ID, issues may arise.

TestFlight has been known to have occasional bugs, which have been present for several years. The platform's restrictions on multiple Apple IDs or devices might be contributing to these issues. To address the "The invitation has already been redeemed" error, consider the aforementioned solutions involving internal and external testing configurations. It is recommended to explore these troubleshooting steps to resolve the problem successfully.