Remove duplicates from downloaded message list (#650)

Some downloaded feeds contain multiple entries for the same message.
See `http://feeds.feedburner.com/abseilio` for an example that has:

```
          <pubDate>2020-09-11T00:00:00-04:00</pubDate>
          <link>https://abseil.io/tips/5</link>
--
          <pubDate>2020-06-01T00:00:00-04:00</pubDate>
          <link>https://abseil.io/tips/5</link>
```

When updating the database for the first time, both messages end up
stored. The following feed updates result in one entry matching the
database, and the second entry having a different creation date. This
makes the second entry always marked as unread.

src/librssguard/core/feeddownloader.cpp

@@ -348,6 +348,8 @@ void FeedDownloader::updateOneFeed(ServiceRoot* acc,
       }
     }
 
+    removeDuplicateMessages(msgs);
+
     // Now make sure, that messages are actually stored to SQL in a locked state.
     qDebugNN << LOGSEC_FEEDDOWNLOADER << "Saving messages of feed ID '"
              << feed->customId() << "' URL: '" << feed->source() << "' title: '" << feed->title() << "' in thread: '"
@@ -418,6 +420,59 @@ bool FeedDownloader::isCacheSynchronizationRunning() const {
   return m_isCacheSynchronizationRunning;
 }
 
+void FeedDownloader::removeDuplicateMessages(QList<Message>& messages) {
+  auto idx = 0;
+  while (idx < messages.size()) {
+    Message& message = messages[idx];
+    std::function<bool(const Message& a, const Message& b)> is_duplicate;
+    if (message.m_id > 0) {
+      is_duplicate = [](const Message& a, const Message& b) {
+        return a.m_id == b.m_id;
+      };
+    }
+    else if (message.m_customId.isEmpty()) {
+      is_duplicate = [](const Message& a, const Message& b) {
+        return std::tie(a.m_title, a.m_url, a.m_author) == std::tie(b.m_title, b.m_url, b.m_author);
+      };
+    }
+    else {
+      is_duplicate = [](const Message& a, const Message& b) {
+        return a.m_customId == b.m_customId;
+      };
+    }
+    auto next_idx = idx + 1; // Index of next message to check after removing all duplicates.
+    auto last_idx = idx; // Index of the last kept duplicate.
+    idx = next_idx;
+
+    // Remove all duplicate messages, and keep the message with the latest created date.
+    // If the created date is identical for all duplicate messages then keep the last message in the list.
+    while (idx < messages.size()) {
+      auto& last_duplicate = messages[last_idx];
+      if (is_duplicate(last_duplicate, messages[idx])) {
+        if (last_duplicate.m_created <= messages[idx].m_created) {
+          // The last seen message was created earlier or at the same date -- keep the current, and remove the last.
+          messages.removeAt(last_idx);
+          if (last_idx + 1 == next_idx) {
+            // The `next_idx` was pointing to the message following the duplicate. With that duplicate removed the
+            // next index needs to be adjusted.
+            next_idx = last_idx;
+          }
+          last_idx = idx;
+          ++idx;
+        }
+        else {
+          messages.removeAt(idx);
+        }
+      }
+      else {
+        ++idx;
+      }
+    }
+
+    idx = next_idx;
+  }
+}
+
 QString FeedDownloadResults::overview(int how_many_feeds) const {
   QStringList result;
 

src/librssguard/core/feeddownloader.h

@@ -59,6 +59,7 @@ class FeedDownloader : public QObject {
                        const QHash<ServiceRoot::BagOfMessages, QStringList>& stated_messages,
                        const QHash<QString, QStringList>& tagged_messages);
     void finalizeUpdate();
+    static void removeDuplicateMessages(QList<Message>& messages);
 
     bool m_isCacheSynchronizationRunning;
     bool m_stopCacheSynchronization;