Fix ObjectStorage#changed_mounts to prevent multiple schedules for one file uploads

ee/app/uploaders/object_storage.rb

@@ -88,7 +88,7 @@ module ObjectStorage
     def changed_mounts
       self.class.uploaders.select do |mount, uploader_class|
         mounted_as = uploader_class.serialization_column(self.class, mount)
-        mount if send(:"#{mounted_as}_changed?") # rubocop:disable GitlabSecurity/PublicSend
+        mount if send(:"#{mounted_as}_changed?") && send(:"#{mounted_as}")&.exists? # rubocop:disable GitlabSecurity/PublicSend
       end.keys
     end
 

ee/spec/models/lfs_object_spec.rb

@@ -61,10 +61,24 @@ describe LfsObject do
           end
 
           it 'schedules the model for migration' do
-            expect(ObjectStorage::BackgroundMoveWorker).to receive(:perform_async).with('LfsObjectUploader', described_class.name, :file, kind_of(Numeric))
+            expect(ObjectStorage::BackgroundMoveWorker)
+              .to receive(:perform_async)
+              .with('LfsObjectUploader', described_class.name, :file, kind_of(Numeric))
+              .once
 
             subject
           end
+
+          it 'schedules the model for migration once' do
+            expect(ObjectStorage::BackgroundMoveWorker)
+              .to receive(:perform_async)
+              .with('LfsObjectUploader', described_class.name, :file, kind_of(Numeric))
+              .once
+
+            lfs_object = create(:lfs_object)
+            lfs_object.file = fixture_file_upload(Rails.root + "spec/fixtures/dk.png", "`/png")
+            lfs_object.save!
+          end
         end
 
         context 'when is unlicensed' do