Fix race condition causing duplicate replies

We check if the reply exists before adding it because
it may have been added already from polling requests

app/assets/javascripts/notes/stores/mutations.js

@@ -33,10 +33,11 @@ export default {
   },
 
   [types.ADD_NEW_REPLY_TO_DISCUSSION](state, note) {
-    const noteObj = utils.findNoteObjectById(state.discussions, note.discussion_id);
+    const discussion = utils.findNoteObjectById(state.discussions, note.discussion_id);
+    const existingNote = discussion && utils.findNoteObjectById(discussion.notes, note.id);
 
-    if (noteObj) {
-      noteObj.notes.push(note);
+    if (discussion && !existingNote) {
+      discussion.notes.push(note);
     }
   },
 

changelogs/unreleased/23400-fix-duplicate-replies.yml

+---
+title: Fix new discussion replies sometimes showing up twice
+merge_request: 17255
+author:
+type: fixed

spec/javascripts/notes/stores/mutation_spec.js

@@ -48,9 +48,22 @@ describe('Notes Store mutations', () => {
   });
 
   describe('ADD_NEW_REPLY_TO_DISCUSSION', () => {
+    const newReply = Object.assign({}, note, { discussion_id: discussionMock.id });
+
+    let state;
+
+    beforeEach(() => {
+      state = { discussions: [{ ...discussionMock }] };
+    });
+
     it('should add a reply to a specific discussion', () => {
-      const state = { discussions: [discussionMock] };
-      const newReply = Object.assign({}, note, { discussion_id: discussionMock.id });
+      mutations.ADD_NEW_REPLY_TO_DISCUSSION(state, newReply);
+
+      expect(state.discussions[0].notes.length).toEqual(4);
+    });
+
+    it('should not add the note if it already exists in the discussion', () => {
+      mutations.ADD_NEW_REPLY_TO_DISCUSSION(state, newReply);
       mutations.ADD_NEW_REPLY_TO_DISCUSSION(state, newReply);
 
       expect(state.discussions[0].notes.length).toEqual(4);