IMprove MR validation logic

app/models/merge_request.rb

@@ -92,7 +92,10 @@ class MergeRequest < ActiveRecord::Base
       errors.add :branch_conflict, "You can not use same branch for source and target branches"
     end
 
-    if self.project.merge_requests.where(source_branch: source_branch, target_branch: target_branch).opened.any?
+    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
+
+    if similar_mrs.any?
       errors.add :base, "There is already an open merge request for this branches"
     end
   end