[MM-45726] Fix Sentry crash: nil deference in app/post_helpers.go:85 (#20654)

* Avoid adding duplicate post

store/sqlstore/post_store.go

@@ -784,6 +784,13 @@ func (s *SqlPostStore) Get(ctx context.Context, id string, opts model.GetPostsOp
 		}
 
 		for _, p := range posts {
+			if p.Id == id {
+				// Based on the conditions above such as sq.Or{ sq.Eq{"p.Id": rootId}, sq.Eq{"p.RootId": rootId}, }
+				// posts may contain the "id" post which has already been fetched and added in the "pl"
+				// So, skip the "id" to avoid duplicate entry of the post
+				continue
+			}
+
 			pl.AddPost(p)
 			pl.AddOrder(p.Id)
 		}

store/storetest/post_store.go

@@ -643,7 +643,7 @@ func testPostStoreGetForThread(t *testing.T, ss store.Store) {
 		}
 		r1, err = ss.Post().Get(context.Background(), o1.Id, opts, o1.UserId, map[string]bool{})
 		require.NoError(t, err)
-		assert.Len(t, r1.Order, 3) // including the root post
+		assert.Len(t, r1.Order, 2) // including the root post
 		assert.True(t, r1.HasNext)
 
 		lastPostID = r1.Order[len(r1.Order)-1]
@@ -675,7 +675,7 @@ func testPostStoreGetForThread(t *testing.T, ss store.Store) {
 		}
 		r1, err = ss.Post().Get(context.Background(), o1.Id, opts, o1.UserId, map[string]bool{})
 		require.NoError(t, err)
-		assert.Len(t, r1.Order, 3) // including the root post
+		assert.Len(t, r1.Order, 2) // including the root post
 		assert.LessOrEqual(t, r1.Posts[r1.Order[1]].CreateAt, firstPostCreateAt)
 		assert.False(t, r1.HasNext)