Filter out duplicate IDs in timelines reducer (#5417)

Possibly the cause of #5379, #5377

app/javascript/mastodon/reducers/timelines.js

@@ -31,10 +31,10 @@ const initialTimeline = ImmutableMap({
 });
 
 const normalizeTimeline = (state, timeline, statuses, next) => {
-  const ids       = ImmutableList(statuses.map(status => status.get('id')));
+  const oldIds    = state.getIn([timeline, 'items'], ImmutableList());
+  const ids       = ImmutableList(statuses.map(status => status.get('id'))).filter(newId => !oldIds.includes(newId));
   const wasLoaded = state.getIn([timeline, 'loaded']);
   const hadNext   = state.getIn([timeline, 'next']);
-  const oldIds    = state.getIn([timeline, 'items'], ImmutableList());
 
   return state.update(timeline, initialTimeline, map => map.withMutations(mMap => {
     mMap.set('loaded', true);
@@ -45,8 +45,8 @@ const normalizeTimeline = (state, timeline, statuses, next) => {
 };
 
 const appendNormalizedTimeline = (state, timeline, statuses, next) => {
-  const ids    = ImmutableList(statuses.map(status => status.get('id')));
   const oldIds = state.getIn([timeline, 'items'], ImmutableList());
+  const ids    = ImmutableList(statuses.map(status => status.get('id'))).filter(newId => !oldIds.includes(newId));
 
   return state.update(timeline, initialTimeline, map => map.withMutations(mMap => {
     mMap.set('isLoading', false);