unique recipient list

lib/emailservice.js

@@ -124,8 +124,11 @@ EmailService.prototype._getRecipientsList = function(notification, emailType, cb
     if (err) return cb(err);
     if (_.isEmpty(preferences)) return cb(null, []);
 
+    var usedEmails = {};
     var recipients = _.compact(_.map(preferences, function(p) {
-      if (!p.email) return;
+      if (!p.email || usedEmails[p.email]) return;
+
+      usedEmails[p.email] = true;
       if (notification.creatorId == p.copayerId && !emailType.notifyDoer) return;
       return {
         copayerId: p.copayerId,