Fix wrong message when adding same epic to issue

Fix misleading success message when adding a
duplicate issue to epic via quick action.

ee/changelogs/unreleased/issue_33222.yml

+---
+title: 'Show the proper message when adding a duplicate issue to an epic'
+merge_request:
+author: 20175
+type: fixed

ee/lib/ee/gitlab/quick_actions/issue_actions.rb

@@ -18,12 +18,19 @@ module EE
           params '<&epic | group&epic | Epic URL>'
           command :epic do |epic_param|
             epic = extract_epic(epic_param)
+            issue = quick_action_target
 
+            message =
               if epic && current_user.can?(:read_epic, epic)
+                if issue&.epic == epic
+                  _('Issue %{issue_reference} has already been added to epic %{epic_reference}.') %
+                    { issue_reference: issue.to_reference, epic_reference: epic.to_reference }
+                else
                   @updates[:epic] = epic
-              message = _('Added an issue to an epic.')
+                  _('Added an issue to an epic.')
+                end
               else
-              message = _("This epic does not exist or you don't have sufficient permission.")
+                _("This epic does not exist or you don't have sufficient permission.")
               end
 
             @execution_message[:epic] = message

ee/spec/services/quick_actions/interpret_service_spec.rb

@@ -241,6 +241,17 @@ describe QuickActions::InterpretService do
               expect(updates).to be_empty
             end
           end
+
+          context 'when issue is already added to epic' do
+            it 'returns error message' do
+              issue = create(:issue, project: project, epic: epic)
+
+              _, updates, message = service.execute(content, issue)
+
+              expect(updates).to be_empty
+              expect(message).to eq("Issue #{issue.to_reference} has already been added to epic #{epic.to_reference}.")
+            end
+          end
         end
 
         context 'when epic does not exist' do

locale/gitlab.pot

@@ -9650,6 +9650,9 @@ msgstr ""
 msgid "Issue"
 msgstr ""
 
+msgid "Issue %{issue_reference} has already been added to epic %{epic_reference}."
+msgstr ""
+
 msgid "Issue Boards"
 msgstr ""