[RN] Make the calendar list distinct

react/features/calendar-sync/middleware.js

 // @flow
 
+import md5 from 'js-md5';
 import RNCalendarEvents from 'react-native-calendar-events';
 
 import { APP_WILL_MOUNT } from '../app';
 }
 
 /**
- * Updates the calendar entries in redux when new list is received.
+ * Updates the calendar entries in redux when new list is received. This
+ * function also dedupes the list of entries based on exact match for title, URL
+ * and time of the day and then sorts by time and limits the list
+ * to MAX_LIST_LENGTH.
  *
  * XXX The function's {@code this} is the redux store.
  *
     if (events && events.length) {
         // eslint-disable-next-line no-invalid-this
         const { dispatch, getState } = this;
-
         const knownDomains = getState()['features/base/known-domains'];
-        const eventList = [];
-
         const now = Date.now();
+        const entryMap = new Map();
 
         for (const event of events) {
             const calendarEntry = _parseCalendarEntry(event, knownDomains);
 
-            calendarEntry
-                && calendarEntry.endDate > now
-                && eventList.push(calendarEntry);
+            if (calendarEntry && calendarEntry.endDate > now) {
+                // This is the data structure we'll try to match all entries to.
+                // The smaller the better, hence the single letter field names.
+                const eventMatchHash = md5.hex(JSON.stringify({
+                    d: new Date(calendarEntry.startDate).toTimeString(),
+                    t: calendarEntry.title,
+                    u: calendarEntry.url
+                }));
+                const existingEntry = entryMap.get(eventMatchHash);
+
+                // We need to dedupe the list based on title, URL and time of
+                // the day, and only show the first occurence. So if there was
+                // no matcing entry or there was, but its a later event, we
+                // overwrite/save it in the map.
+                if (!existingEntry
+                    || existingEntry.startDate > event.startDate) {
+                    entryMap.set(eventMatchHash, calendarEntry);
+                }
+            }
         }
 
         dispatch(
             setCalendarEvents(
-                eventList
+                Array.from(entryMap.values())
                     .sort((a, b) => a.startDate - b.startDate)
-                    .slice(0, MAX_LIST_LENGTH)));
+                    .slice(0, MAX_LIST_LENGTH)
+            ));
     }
 }