This is a correct observation. When trying to add a user with the same email address that has been used before, a red circle with an "x" will appear behind it, indicating you cannot use that email address.
When you remove a user from the user management section, the account connected to that e-mail will still remain stored in the backend.
The e-mail address is a unique ID and therefore be used twice.
If you have accidentally removed a user and, would like to re-use the email address, please contact firstname.lastname@example.org.
We will remove the account from the backend for you, so you may re-use it.