Merge branch 'duplicate_notifications_fix' into 'master'

Fixed duplicated issue note email notifications.

Fixes #2560 



See issue for the details.

Without `uniq` modified tests were failing with:
```
     Failure/Error: notification.new_note(note)
       (Notify (class)).note_issue_email(21, 1)
           expected: 1 time with arguments: (21, 1)
           received: 2 times with arguments: (21, 1)
     # /home/bak1an/.rvm/gems/ruby-2.1.6@gitlab/gems/sidekiq-3.3.0/lib/sidekiq/extensions/action_mailer.rb:17:in `public_send'
     # /home/bak1an/.rvm/gems/ruby-2.1.6@gitlab/gems/sidekiq-3.3.0/lib/sidekiq/extensions/action_mailer.rb:17:in `perform'
     # /home/bak1an/.rvm/gems/ruby-2.1.6@gitlab/gems/sidekiq-3.3.0/lib/sidekiq/testing.rb:74:in `block in raw_push'
     # /home/bak1an/.rvm/gems/ruby-2.1.6@gitlab/gems/sidekiq-3.3.0/lib/sidekiq/testing.rb:69:in `each'
     # /home/bak1an/.rvm/gems/ruby-2.1.6@gitlab/gems/sidekiq-3.3.0/lib/sidekiq/testing.rb:69:in `raw_push'
     # /home/bak1an/.rvm/gems/ruby-2.1.6@gitlab/gems/sidekiq-3.3.0/lib/sidekiq/client.rb:68:in `push'
     # /home/bak1an/.rvm/gems/ruby-2.1.6@gitlab/gems/sidekiq-3.3.0/lib/sidekiq/worker.rb:85:in `client_push'
     # /home/bak1an/.rvm/gems/ruby-2.1.6@gitlab/gems/sidekiq-3.3.0/lib/sidekiq/extensions/generic_proxy.rb:19:in `method_missing'
     # ./app/services/notification_service.rb:144:in `block in new_note'
     # ./app/services/notification_service.rb:143:in `each'
     # ./app/services/notification_service.rb:143:in `new_note'
     # ./spec/services/notification_service_spec.rb:63:in `block (5 levels) in <top (required)>'
```

I have also added `once` to all `should_email` checks within `notification_service_spec.rb` since it's probably the correct behavior to notify users only once on the same event. Nothing else failed out of the box but we can keep these assertions for future.

See merge request !1925

CHANGELOG

@@ -23,6 +23,7 @@ v 8.2.2
   - Prevent "413 Request entity too large" errors when pushing large files with LFS
   - Fix invalid links within projects dashboard header
   - Make current user the first user in assignee dropdown in issues detail page (Stan Hu)
+  - Fix: duplicate email notifications on issue comments
 v 8.2.1
   - Forcefully update builds that didn't want to update with state machine

app/services/notification_service.rb

@@ -145,6 +145,7 @@ class NotificationService
     recipients = reject_unsubscribed_users(recipients, note.noteable)
     recipients.delete(note.author)
+    recipients = recipients.uniq
     # build notify method like 'note_commit_email'
     notify_method = "note_#{note.noteable_type.underscore}_email".to_sym

spec/services/notification_service_spec.rb

@@ -45,6 +45,7 @@ describe NotificationService do
         project.team << [issue.author, :master]
         project.team << [issue.assignee, :master]
         project.team << [note.author, :master]
+        create(:note_on_issue, noteable: issue, project_id: issue.project_id, note: '@subscribed_participant cc this guy')
       end
       describe :new_note do
@@ -60,6 +61,7 @@ describe NotificationService do
           should_email(note.noteable.assignee)
           should_email(@u_mentioned)
           should_email(@subscriber)
+          should_email(@subscribed_participant)
           should_not_email(note.author)
           should_not_email(@u_participating)
           should_not_email(@u_disabled)
@@ -381,18 +383,19 @@ describe NotificationService do
   def add_users_with_subscription(project, issuable)
     @subscriber = create :user
     @unsubscriber = create :user
+    @subscribed_participant = create(:user, username: 'subscribed_participant', notification_level: Notification::N_PARTICIPATING)
+    project.team << [@subscribed_participant, :master]
     project.team << [@subscriber, :master]
     project.team << [@unsubscriber, :master]
     issuable.subscriptions.create(user: @subscriber, subscribed: true)
+    issuable.subscriptions.create(user: @subscribed_participant, subscribed: true)
     issuable.subscriptions.create(user: @unsubscriber, subscribed: false)
   end
   def sent_to_user?(user)
-    ActionMailer::Base.deliveries.any? do |message|
-      message.to.include?(user.email)
-    end
+    ActionMailer::Base.deliveries.map(&:to).flatten.count(user.email) == 1
   end
   def should_email(user)