Improve error message when attempting to reopen MR and there's a new open MR for the same branch

app/controllers/projects/merge_requests_controller.rb

@@ -122,6 +122,10 @@ class Projects::MergeRequestsController < Projects::MergeRequests::ApplicationCo
 
     respond_to do |format|
       format.html do
+        if merge_request_params[:state_event] && @merge_request.errors.any?
+          flash[:alert] = @merge_request.errors.values.flatten.to_sentence
+        end
+
         if @merge_request.valid?
           redirect_to([@merge_request.target_project.namespace.becomes(Namespace), @merge_request.target_project, @merge_request])
         else

app/models/merge_request.rb

@@ -547,7 +547,7 @@ class MergeRequest < ActiveRecord::Base
       similar_mrs = similar_mrs.where('id not in (?)', self.id) if self.id
       if similar_mrs.any?
         errors.add :validate_branches,
-                   "Cannot Create: This merge request already exists: #{similar_mrs.pluck(:title)}"
+                   "Cannot open this Merge Request because another open Merge Request already exists for this source branch: #{similar_mrs.first.title}"
       end
     end
   end

spec/controllers/projects/merge_requests_controller_spec.rb

@@ -290,6 +290,18 @@ describe Projects::MergeRequestsController do
 
       it_behaves_like 'update invalid issuable', MergeRequest
     end
+
+    context 'there are two merge requests with the same source branch' do
+      it "does not allow to reopen a closed merge request if another one is open" do
+        merge_request.close!
+        create(:merge_request, source_project: merge_request.source_project, source_branch: merge_request.source_branch)
+
+        update_merge_request(state_event: 'reopen')
+
+        expect(controller).to set_flash[:alert].to(/another open Merge Request already exists for this source branch/)
+        expect(merge_request.reload).to be_closed
+      end
+    end
   end
 
   describe 'POST merge' do