Fix swimlanes duplicate epics

Unite them by using `epic.id`

ee/app/assets/javascripts/boards/stores/mutations.js

 import Vue from 'vue';
-import { union } from 'lodash';
+import { union, unionBy } from 'lodash';
 import mutationsCE, { addIssueToList, removeIssueFromList } from '~/boards/stores/mutations';
 import { moveIssueListHelper } from '~/boards/boards_util';
 import { s__ } from '~/locale';
@@ -130,7 +130,7 @@ export default {
   },
 
   [mutationTypes.RECEIVE_EPICS_SUCCESS]: (state, epics) => {
-    Vue.set(state, 'epics', union(state.epics || [], epics));
+    Vue.set(state, 'epics', unionBy(state.epics || [], epics, 'id'));
   },
 
   [mutationTypes.RESET_EPICS]: state => {

ee/changelogs/unreleased/swimlane-dups.yml

+---
+title: Fix swimlanes duplicate epics
+merge_request: 44728
+author:
+type: fixed

ee/spec/frontend/boards/stores/mutations_spec.js

@@ -232,6 +232,17 @@ describe('RECEIVE_EPICS_SUCCESS', () => {
 
     expect(state.epics).toEqual(mockEpics);
   });
+
+  it("doesn't add duplicate epics", () => {
+    state = {
+      ...state,
+      epics: mockEpics,
+    };
+
+    mutations.RECEIVE_EPICS_SUCCESS(state, mockEpics);
+
+    expect(state.epics).toEqual(mockEpics);
+  });
 });
 
 describe('RESET_EPICS', () => {