"Email already in use" error

In lieu of not having Shadow Viewer, I have been making great use of the new feature "Specify topic author" and have been using that quite successfully too. As I have all of our Agents into the system, when I want to add a (private) idea that was submitted by one of them, I just click Specify topic author and put in their email address. The system then associates that Topic to that author. It works great.

Until it doesn't, that is. One of the agents I put in allowed me to "Specify topic author" once, but now each time I try and use his email again, I get "Email already in use" error.

When I do this same thing with other authors in the system, it does not give me this error.

I should also note the author in question does have the ability to see private objects, but is not part of a defined Department.