Looking at the information about staged users, I think everything works as designed. But maybe no one thought about the case where the email address will be used as a second one.
Staged users are created when you send an email which becomes a message in the discourse forum. For example, sending an email to team@discourse.org will automatically be converted to a personal message to the @team.
I’m not sure if you have to register with the second email address before the two accounts can be merged. But someone else will be able to help you.