Don't link LFS-objects multiple times.

If Unlinking a fork would fail somewhere after this, the LFS objects
might still be linked. Which would cause issues when trying to destroy
a project.

app/services/projects/unlink_fork_service.rb

@@ -5,7 +5,7 @@ module Projects
 
       if fork_source = @project.fork_source
         fork_source.lfs_objects.find_each do |lfs_object|
-          lfs_object.projects << @project
+          lfs_object.projects << @project unless lfs_object.projects.include?(@project)
         end
 
         refresh_forks_count(fork_source)

changelogs/unreleased/bvl-fix-unlinking-with-lfs-objects.yml

+---
+title: Don't link LFS objects to a project when unlinking forks when they were already
+  linked
+merge_request: 16006
+author:
+type: fixed

spec/services/projects/unlink_fork_service_spec.rb

@@ -62,6 +62,26 @@ describe Projects::UnlinkForkService do
     expect(source.forks_count).to be_zero
   end
 
+  context 'when the source has LFS objects' do
+    let(:lfs_object) { create(:lfs_object) }
+
+    before do
+      lfs_object.projects << project
+    end
+
+    it 'links the fork to the lfs object before unlinking' do
+      subject.execute
+
+      expect(lfs_object.projects).to include(forked_project)
+    end
+
+    it 'does not fail if the lfs objects were already linked' do
+      lfs_object.projects << forked_project
+
+      expect { subject.execute }.not_to raise_error
+    end
+  end
+
   context 'when the original project was deleted' do
     it 'does not fail when the original project is deleted' do
       source = forked_project.forked_from_project