code: protected boolean isAlreadyAvailable(String userName) {
  
          List<CmsUser> availableUsers;
          try {
              availableUsers = OpenCms.getOrgUnitManager().getUsers(getCms(), m_ou, false);
          } catch (CmsException e) {
              availableUsers = new ArrayList<CmsUser>();
          }
          Iterator<CmsUser> itAvailableUsers = availableUsers.iterator();
          while (itAvailableUsers.hasNext()) {
              if (userName.equals(itAvailableUsers.next().getSimpleName())) {
                  return true;
              }
          }
          return false;
      }
docs: Checks if the given user name is already available inside the current ou.<p>
  
  @param userName the user name to check
  @return <code>true</code> if the user name is already available, otherwise return <code>false</code>