Better error message when entering duplicate domains for automatic group membership

Some minor glitches related to managing groups:

  1. when you try to add an email domain to the list for automatic membership and that domain already exists, it nevertheless says “No matches found”. If you then go on and add that domain, it is accepted without error message. But when you then save the whole form it throws an “XYZ is not a valid domain error”.
  2. When adding a new member to a group (manually, via the UI), immediately after adding, the new member appears twice on the members list.