make mailings unique, fix query that retrieves the Users for a mailing
make mailings unique, fix query that retrieves the Users for a mailing

ramonh/rgsoc-teams,TeamCheesy/rgsoc-teams,TeamCheesy/rgsoc-teams,ramonh/rgsoc-teams,michaelem/rgsoc-teams,beanieboi/rgsoc-teams,rails-girls-summer-of-code/rgsoc-teams,beanieboi/rgsoc-teams,ramonh/rgsoc-teams,TeamCheesy/rgsoc-teams,michaelem/rgsoc-teams,michaelem/rgsoc-teams,beanieboi/rgsoc-teams,rails-girls-summer-of-code/rgsoc-teams,rails-girls-summer-of-code/rgsoc-teams