If a preferred reviewer is added before co-authors are added, then the same person who was added as a preferred reviewer can be added as a co-author too if the persons' name is slightly different.
For example, if you add reviewer first with email xy@xy.z and First Name X, Last Name Y and then go to the author step and try to add the same information for a co-author you would not be able to do so.
However, if you try to enter the email address xy@xy.z and First Name XZ, Last Name Y, you would be able to add them as a co-author.
It seems that on the Author's step the system is comparing both email address and name and if they all do not match it allows adding a person that is already added as a recommended reviewer.
