Is it possible for a portal user to have the same username in two separate orgs.  For example, Company A has a portal and Mike Smith chose msmith@gmail.com as the username.  Company B also has a portal and Mike Smith needs to be a portal user in Company B as well and wants to choose msmith@gmail.com as the username.  Is this possible?  The reason I ask is that we're getting the following error message in one of our customer's portals, even though we have check the user table before registration to catch the error if the user tries to use a username that already exists in the user table.

There was an error in registering a user in site <PORTAL_NAME_HERE>. The error message is: Duplicate Username.<br>Another user has already selected this username.<br>Please select another.