Workgroup manager user name list

Hey folks, A 10.3.9 OSXS Xserve has two individuals set up to log in and admin the machine. When using the Workgroup manager (my version is 10.4.2) to admin the server, I cannot add in the same name as a user for an email account. Is this something well-known, but not in the docs I've read: that one cannot create a username in the workgroup manager that is identical to a username for logging into the server? In searching Apple's kb, I haven't found an answer.