Detect already enabled DeployKeys in EnableDeployKeyService

Ensures deploy keys can't be re-added, which causes a validation error

app/services/projects/enable_deploy_key_service.rb

@@ -4,7 +4,10 @@ module Projects
       key = accessible_keys.find_by(id: params[:key_id] || params[:id])
       return unless key
 
+      unless project.deploy_keys.include?(key)
         project.deploy_keys << key
+      end
+
       key
     end
 

changelogs/unreleased/29673-500-internal-server-error-when-enabling-a-deploy-key-more-than-once-through-api.yml

+---
+title: Detect already enabled DeployKeys in EnableDeployKeyService
+merge_request:
+author:

spec/services/projects/enable_deploy_key_service_spec.rb

@@ -21,6 +21,16 @@ describe Projects::EnableDeployKeyService, services: true do
     end
   end
 
+  context 'add the same key twice' do
+    before do
+      project.deploy_keys << deploy_key
+    end
+
+    it 'returns existing key' do
+      expect(service.execute).to eq(deploy_key)
+    end
+  end
+
   def service
     Projects::EnableDeployKeyService.new(project, user, params)
   end