diff --git a/CHANGELOG b/CHANGELOG
index 2d57e013a9e11114de55695f58d6e2cc4a54b010..26de1642b3b4eea962518c74645054efa224e28e 100644
--- a/CHANGELOG
+++ b/CHANGELOG
@@ -49,6 +49,7 @@ v 8.5.0 (unreleased)
   - Fixed logo animation on Safari (Roman Rott)
   - Hide remove source branch button when the MR is merged but new commits are pushed (Zeger-Jan van de Weg)
   - In seach autocomplete show only groups and projects you are member of
+  - Don't process cross-reference notes from forks
   - Fix: init.d script not working on OS X
   - Faster snippet search
   - Title for milestones should be unique (Zeger-Jan van de Weg)
diff --git a/app/services/system_note_service.rb b/app/services/system_note_service.rb
index 1083bcec05414e3f5664f30c2d58f07362dac75c..edced010811b9b9a7fe5a4579751a0d1755d8a09 100644
--- a/app/services/system_note_service.rb
+++ b/app/services/system_note_service.rb
@@ -274,12 +274,15 @@ class SystemNoteService
   # Check if a cross reference to a noteable from a mentioner already exists
   #
   # This method is used to prevent multiple notes being created for a mention
-  # when a issue is updated, for example.
+  # when a issue is updated, for example. The method also calls notes_for_mentioner
+  # to check if the mentioner is a commit, and return matches only on commit hash
+  # instead of project + commit, to avoid repeated mentions from forks.
   #
   # noteable  - Noteable object being referenced
   # mentioner - Mentionable object
   #
   # Returns Boolean
+
   def self.cross_reference_exists?(noteable, mentioner)
     # Initial scope should be system notes of this noteable type
     notes = Note.system.where(noteable_type: noteable.class)
@@ -291,14 +294,20 @@ class SystemNoteService
       notes = notes.where(noteable_id: noteable.id)
     end
 
-    gfm_reference = mentioner.gfm_reference(noteable.project)
-    notes = notes.where(note: cross_reference_note_content(gfm_reference))
-
-    notes.count > 0
+    notes_for_mentioner(mentioner, noteable, notes).count > 0
   end
 
   private
 
+  def self.notes_for_mentioner(mentioner, noteable, notes)
+    if mentioner.is_a?(Commit)
+      notes.where('note LIKE ?', "#{cross_reference_note_prefix}%#{mentioner.to_reference(nil)}")
+    else
+      gfm_reference = mentioner.gfm_reference(noteable.project)
+      notes.where(note: cross_reference_note_content(gfm_reference))
+    end
+  end
+
   def self.create_note(args = {})
     Note.create(args.merge(system: true))
   end
diff --git a/spec/services/system_note_service_spec.rb b/spec/services/system_note_service_spec.rb
index d3364a71022c9d953c59971ad4570dfeea2066b3..1bdc03af12d7baa18f6891ecc6e56b7a58be6b9b 100644
--- a/spec/services/system_note_service_spec.rb
+++ b/spec/services/system_note_service_spec.rb
@@ -424,6 +424,21 @@ describe SystemNoteService, services: true do
           to be_falsey
       end
     end
+
+    context 'commit with cross-reference from fork' do
+      let(:author2) { create(:user) }
+      let(:forked_project) { Projects::ForkService.new(project, author2).execute }
+      let(:commit2) { forked_project.commit }
+
+      before do
+        described_class.cross_reference(noteable, commit0, author2)
+      end
+
+      it 'is true when a fork mentions an external issue' do
+        expect(described_class.cross_reference_exists?(noteable, commit2)).
+            to be true
+      end
+    end
   end
 
   include JiraServiceHelper