Existing network user can't join another site on same network

Hello. When logged on as an existing user (non admin) on a network install, whenever I try to register on another site on the same network, I am not allowed to join. I get a double error saying the username is already in use and the email is already in use. Is this by design? Or is it an error? What can be done to work around this?

To see an image of the screenshot showing the error messages, click here.