Prevent duplicate mention notifications when the post is received twice

closes #7721

Changelog.md

@@ -5,6 +5,7 @@
 * Make script/server work on readonly filesystems [#7719](https://github.com/diaspora/diaspora/pull/7719)
 
 ## Bug fixes
+* Prevent duplicate mention notifications when the post is received twice [#7721](https://github.com/diaspora/diaspora/pull/7721)
 
 ## Features
 * Add basic html5 audio/video embedding support [#6418](https://github.com/diaspora/diaspora/pull/6418)

app/models/notifications/mentioned.rb

@@ -18,7 +18,10 @@ module Notifications
         )
 
         relevant_mentions.each do |mention|
-          create_notification(mention.person.owner, mention, actor).try(:email_the_user, mention, actor)
+          recipient = mention.person.owner
+          unless exists?(recipient: recipient, target: mention)
+            create_notification(recipient, mention, actor).try(:email_the_user, mention, actor)
+          end
         end
       end
     end

spec/models/notifications/mentioned_spec.rb

@@ -51,5 +51,17 @@ describe Notifications::Mentioned do
       expect(TestNotification).not_to receive(:create_notification)
       TestNotification.notify(status_message, nil)
     end
+
+    it "doesn't create notification if it already exists" do
+      status_message = FactoryGirl.create(:status_message, text: text_mentioning(alice), author: eve.person)
+      TestNotification.create(
+        recipient: alice,
+        target:    Mention.where(mentions_container: status_message, person: alice.person_id).first,
+        actors:    [status_message.author]
+      )
+
+      expect(TestNotification).not_to receive(:create_notification)
+      TestNotification.notify(status_message, nil)
+    end
   end
 end