Add existing organisation name validation

app/controllers/merge_requests_controller.rb

@@ -150,6 +150,8 @@ private
     when "new_organisation_name"
       if merge_request_params[:new_organisation_name].blank?
         @merge_request.errors.add(:new_organisation_name, I18n.t("validations.merge_request.new_organisation_name_blank"))
+      elsif Organisation.where("lower(name) = ?", merge_request_params[:new_organisation_name].downcase).exists?
+        @merge_request.errors.add(:new_organisation_name, I18n.t("validations.merge_request.new_organisation_name_exists"))
       end
     end
   end

config/locales/en.yml

@@ -519,6 +519,7 @@ en:
       telephone_number_correct_blank: Select to confirm or enter a new telephone number
       new_telephone_number_blank: Enter a valid telephone number
       new_organisation_name_blank: "Enter an organisation name"
+      new_organisation_name_exists: "An organisation with this name already exists"
 
   soft_validations:
     net_income:

spec/requests/merge_requests_controller_spec.rb

@@ -529,6 +529,29 @@ RSpec.describe MergeRequestsController, type: :request do
             expect { request }.not_to(change { merge_request.reload.attributes })
           end
         end
+
+        context "when the new organisation name already exists" do
+          before do
+            create(:organisation, name: "new org name")
+          end
+
+          let(:params) do
+            { merge_request: { new_organisation_name: "New org name", page: "new_organisation_name" } }
+          end
+
+          let(:request) do
+            patch "/merge-request/#{merge_request.id}", headers:, params:
+          end
+
+          it "renders the error" do
+            request
+            expect(page).to have_content("An organisation with this name already exists")
+          end
+
+          it "does not update the organisation name" do
+            expect { request }.not_to(change { merge_request.reload.attributes })
+          end
+        end
       end
     end
   end