No TestFlight invite email gets sent for internal users

Question

I set up my application for TestFlight. For external users it is waiting for review. But if I want to invite internal users, I select them and press invite. iTunes Connects says

Answer 1

The problem with inviting an internal tester is that you cannot have an internal and external tester with the same apple ID.

Although deleted and recreated the user in user roles I still had the same problem. When I removed the duplicate email from External Testers and went back to Internal Testers, the invite was sent.

Source : Check this SO link

Edit : I will still like to mention that when I tried it then I had to use an email which was not connected to iTunesConnect earlier. For some reason, Apple doesn't allow an existing iTunesConnect User to register as Internal Tester on Testflight and I don't have any explanation for this issue. I believe even Apple doesn't have an explanation and I assume its a major bug ;)