If you try to import twice an xml with project_members user group the second import will fail with following error:
info Add user kat to project.
error UserNotAddedAsProjectMemberException: error: User is already a member
in /usr/share/tuleap/src/common/project/ProjectXMLImporter.class.php L175
This case should be gracefully handled and we shouldn't attempt to add a user that is already member.