You are correct. Nonetheless it is strange that a name for a guest author is rejected on the grounds that there is already an author with that name when there is no such author. (The surname is Smith.) I have found a workaround which is to create a dummy guest author and then to edit that name. The renaming does not get rejected on the edit even if it is rejected on its creation.
I was able to add another guest author. However, the Author’s box does not appear for this author even though it appears for the other guest authors.
I am using the WP theme Twenty Twelve which unfortunately places the authors details below the post in addition to the Author’s box.
