Check for existing MR only for opened/reopened MR

app/models/merge_request.rb

@@ -92,6 +92,7 @@ class MergeRequest < ActiveRecord::Base
       errors.add :branch_conflict, "You can not use same branch for source and target branches"
     end
 
+    if opened? || reopened?
       similar_mrs = self.project.merge_requests.where(source_branch: source_branch, target_branch: target_branch).opened
       similar_mrs = similar_mrs.where('id not in (?)', self.id) if self.id
 
@@ -99,6 +100,7 @@ class MergeRequest < ActiveRecord::Base
         errors.add :base, "There is already an open merge request for this branches"
       end
     end
+  end
 
   def reload_code
     self.reloaded_commits