diff --git a/resources/sql.qrc b/resources/sql.qrc
index 756e5d000..1d36007d5 100755
--- a/resources/sql.qrc
+++ b/resources/sql.qrc
@@ -14,6 +14,7 @@
     <file>sql/db_update_mysql_11_12.sql</file>
     <file>sql/db_update_mysql_12_13.sql</file>
     <file>sql/db_update_mysql_13_14.sql</file>
+    <file>sql/db_update_mysql_14_15.sql</file>
 
     <file>sql/db_init_sqlite.sql</file>
     <file>sql/db_update_sqlite_1_2.sql</file>
@@ -29,5 +30,6 @@
     <file>sql/db_update_sqlite_11_12.sql</file>
     <file>sql/db_update_sqlite_12_13.sql</file>
     <file>sql/db_update_sqlite_13_14.sql</file>
+    <file>sql/db_update_sqlite_14_15.sql</file>
   </qresource>
 </RCC>
\ No newline at end of file
diff --git a/src/librssguard/core/feeddownloader.cpp b/src/librssguard/core/feeddownloader.cpp
index 2614e0046..769baba9b 100644
--- a/src/librssguard/core/feeddownloader.cpp
+++ b/src/librssguard/core/feeddownloader.cpp
@@ -4,6 +4,7 @@
 
 #include "core/messagefilter.h"
 #include "definitions/definitions.h"
+#include "miscellaneous/application.h"
 #include "services/abstract/cacheforserviceroot.h"
 #include "services/abstract/feed.h"
 
@@ -79,11 +80,12 @@ void FeedDownloader::updateOneFeed(Feed* feed) {
                      << QThread::currentThreadId() << "\'.";
 
   bool error_during_obtaining = false;
+  QElapsedTimer tmr; tmr.start();
   QList<Message> msgs = feed->obtainNewMessages(&error_during_obtaining);
 
   qDebug().nospace() << "Downloaded " << msgs.size() << " messages for feed ID "
                      << feed->customId() << " URL: " << feed->url() << " title: " << feed->title() << " in thread: \'"
-                     << QThread::currentThreadId() << "\'.";
+                     << QThread::currentThreadId() << "\'. Operation took " << tmr.nsecsElapsed() / 1000 << " microseconds.";
 
   // Now, sanitize messages (tweak encoding etc.).
   for (auto& msg : msgs) {
@@ -102,43 +104,52 @@ void FeedDownloader::updateOneFeed(Feed* feed) {
   }
 
   if (!feed->messageFilters().isEmpty()) {
+    tmr.restart();
+
+    bool is_main_thread = QThread::currentThread() == qApp->thread();
+    QSqlDatabase database = is_main_thread ?
+                            qApp->database()->connection(metaObject()->className()) :
+                            qApp->database()->connection(QSL("feed_upd"));
+
     // Perform per-message filtering.
     QJSEngine filter_engine;
 
     // Create JavaScript communication wrapper for the message.
-    MessageObject msg_obj;
+    MessageObject msg_obj(&database, feed->customId(), feed->getParentServiceRoot()->accountId());
 
     // Register the wrapper.
     auto js_object = filter_engine.newQObject(&msg_obj);
 
     filter_engine.globalObject().setProperty("msg", js_object);
 
+    qDebug().nospace() << "Setting up JS evaluation took " << tmr.nsecsElapsed() / 1000 << " microseconds.";
+
     for (int i = 0; i < msgs.size(); i++) {
+      tmr.restart();
+
       // Attach live message object to wrapper.
       msg_obj.setMessage(&msgs[i]);
+      qDebug().nospace() << "Hooking message took " << tmr.nsecsElapsed() / 1000 << " microseconds.";
 
       auto feed_filters = feed->messageFilters();
 
-      for (int i = 0; i < feed_filters.size(); i++) {
-        QPointer<MessageFilter> filter = feed_filters.at(i);
+      for (int j = 0; j < feed_filters.size(); j++) {
+        QPointer<MessageFilter> filter = feed_filters.at(j);
 
         if (filter.isNull()) {
           qWarning("Message filter was probably deleted, removing its pointer from list of filters.");
-          feed_filters.removeAt(i--);
+          feed_filters.removeAt(j--);
           continue;
         }
 
         MessageFilter* msg_filter = filter.data();
 
-        // Call the filtering logic, given function must return integer value from
-        // FilteringAction enumeration.
-        //
-        // 1. All Qt properties of MessageObject class are accessible.
-        //    For example msg.title.includes("A") returns true if message's title includes "A" etc.
-        // 2. Some Qt properties of MessageObject are writable, so you can alter your message!
-        //    For example msg.isImportant = true.
+        tmr.restart();
+
         FilteringAction decision = msg_filter->filterMessage(&filter_engine);
 
+        qDebug().nospace() << "Running filter script, it took " << tmr.nsecsElapsed() / 1000 << " microseconds.";
+
         switch (decision) {
           case FilteringAction::Accept:
 
diff --git a/src/librssguard/core/message.cpp b/src/librssguard/core/message.cpp
index b372c3345..045da982e 100644
--- a/src/librssguard/core/message.cpp
+++ b/src/librssguard/core/message.cpp
@@ -4,6 +4,9 @@
 
 #include "miscellaneous/textfactory.h"
 
+#include <QSqlDatabase>
+#include <QSqlError>
+#include <QSqlQuery>
 #include <QVariant>
 
 Enclosure::Enclosure(QString url, QString mime) : m_url(std::move(url)), m_mimeType(std::move(mime)) {}
@@ -129,15 +132,77 @@ uint qHash(const Message& key) {
   return (uint(key.m_accountId) * 10000) + uint(key.m_id);
 }
 
-MessageObject::MessageObject(QObject* parent) : QObject(parent), m_message(nullptr) {}
+MessageObject::MessageObject(QSqlDatabase* db, const QString& feed_custom_id, int account_id, QObject* parent)
+  : QObject(parent), m_db(db), m_feedCustomId(feed_custom_id), m_accountId(account_id), m_message(nullptr) {}
 
 void MessageObject::setMessage(Message* message) {
   m_message = message;
 }
 
 bool MessageObject::isDuplicateWithAttribute(int attribute_check) const {
-  // TODO: Check database according to duplication attribute_check.
-  return int(attribute_check) == 4;
+  if (attribute_check <= 0) {
+    qCritical("Bad DuplicationAttributeCheck value '%d' was passed from JS filter script.", attribute_check);
+    return true;
+  }
+
+  // Check database according to duplication attribute_check.
+  DuplicationAttributeCheck attrs = static_cast<DuplicationAttributeCheck>(attribute_check);
+  QSqlQuery q(*m_db);
+  QStringList where_clauses;
+  QList<QPair<QString, QVariant>> bind_values;
+
+  // Now we construct the query according to parameter.
+  if ((attrs& DuplicationAttributeCheck::SameTitle) == DuplicationAttributeCheck::SameTitle) {
+    where_clauses.append(QSL("title = :title"));
+    bind_values.append({":title", title()});
+  }
+
+  if ((attrs& DuplicationAttributeCheck::SameUrl) == DuplicationAttributeCheck::SameUrl) {
+    where_clauses.append(QSL("url = :url"));
+    bind_values.append({":url", url()});
+  }
+
+  if ((attrs& DuplicationAttributeCheck::SameAuthor) == DuplicationAttributeCheck::SameAuthor) {
+    where_clauses.append(QSL("author = :author"));
+    bind_values.append({":author", author()});
+  }
+
+  if ((attrs& DuplicationAttributeCheck::SameDateCreated) == DuplicationAttributeCheck::SameDateCreated) {
+    where_clauses.append(QSL("date_created = :date_created"));
+    bind_values.append({":date_created", created().toMSecsSinceEpoch()});
+  }
+
+  where_clauses.append(QSL("account_id = :account_id"));
+  bind_values.append({":account_id", accountId()});
+
+  if ((attrs& DuplicationAttributeCheck::AllFeedsSameAccount) != DuplicationAttributeCheck::AllFeedsSameAccount) {
+    // Limit to current feed.
+    where_clauses.append(QSL("feed = :feed"));
+    bind_values.append({":feed", feedCustomId()});
+  }
+
+  QString full_query = QSL("SELECT COUNT(*) FROM Messages WHERE ") + where_clauses.join(QSL(" AND ")) + QSL(";");
+
+  q.setForwardOnly(true);
+  q.prepare(full_query);
+
+  for (const auto& bind : bind_values) {
+    q.bindValue(bind.first, bind.second);
+  }
+
+  if (q.exec() && q.next()) {
+    if (q.record().value(0).toInt() > 0) {
+      // Whoops, we have the "same" message in database.
+      qDebug("Message '%s' was identified as duplicate by filter script.", qPrintable(title()));
+      return true;
+    }
+  }
+  else if (q.lastError().isValid()) {
+    qWarning("Error when checking for duplicate messages via filtering system, error: '%s'.",
+             qPrintable(q.lastError().text()));
+  }
+
+  return false;
 }
 
 QString MessageObject::title() const {
@@ -195,3 +260,11 @@ bool MessageObject::isImportant() const {
 void MessageObject::setIsImportant(bool is_important) {
   m_message->m_isImportant = is_important;
 }
+
+QString MessageObject::feedCustomId() const {
+  return m_feedCustomId;
+}
+
+int MessageObject::accountId() const {
+  return m_accountId;
+}
diff --git a/src/librssguard/core/message.h b/src/librssguard/core/message.h
index 3098b619c..512ce152f 100644
--- a/src/librssguard/core/message.h
+++ b/src/librssguard/core/message.h
@@ -10,6 +10,8 @@
 #include <QSqlRecord>
 #include <QStringList>
 
+class QSqlDatabase;
+
 // Represents single enclosure.
 struct Enclosure {
   public:
@@ -79,15 +81,35 @@ enum class FilteringAction {
 };
 
 enum class DuplicationAttributeCheck {
+  // Message with same title in DB.
   SameTitle = 1,
+
+  // Message with same URL in DB.
   SameUrl = 2,
+
+  // Message with same author in DB.
   SameAuthor = 4,
-  SameFeed = 8,
-  SameDateCreated = 16
+
+  // Messages with same creation date in DB.
+  SameDateCreated = 8,
+
+  // Compare with all messages from the account not only with messages from same feed.
+  AllFeedsSameAccount = 16
 };
 
+inline DuplicationAttributeCheck operator|(DuplicationAttributeCheck lhs, DuplicationAttributeCheck rhs) {
+  return static_cast<DuplicationAttributeCheck>(int(lhs) | int(rhs));
+}
+
+inline DuplicationAttributeCheck operator&(DuplicationAttributeCheck lhs, DuplicationAttributeCheck rhs) {
+  return static_cast<DuplicationAttributeCheck>(int(lhs) & int(rhs));
+}
+
 class MessageObject : public QObject {
   Q_OBJECT
+
+  Q_PROPERTY(QString feedCustomId READ feedCustomId)
+  Q_PROPERTY(int accountId READ accountId)
   Q_PROPERTY(QString title READ title WRITE setTitle)
   Q_PROPERTY(QString url READ url WRITE setUrl)
   Q_PROPERTY(QString author READ author WRITE setAuthor)
@@ -97,7 +119,7 @@ class MessageObject : public QObject {
   Q_PROPERTY(bool isImportant READ isImportant WRITE setIsImportant)
 
   public:
-    explicit MessageObject(QObject* parent = nullptr);
+    explicit MessageObject(QSqlDatabase* db, const QString& feed_custom_id, int account_id, QObject* parent = nullptr);
 
     void setMessage(Message* message);
 
@@ -107,6 +129,9 @@ class MessageObject : public QObject {
     Q_INVOKABLE bool isDuplicateWithAttribute(int attribute_check) const;
 
     // Generic Message's properties bindings.
+    QString feedCustomId() const;
+    int accountId() const;
+
     QString title() const;
     void setTitle(const QString& title);
 
@@ -129,6 +154,9 @@ class MessageObject : public QObject {
     void setIsImportant(bool is_important);
 
   private:
+    QSqlDatabase* m_db;
+    QString m_feedCustomId;
+    int m_accountId;
     Message* m_message;
 };
 
diff --git a/src/librssguard/core/messagefilter.cpp b/src/librssguard/core/messagefilter.cpp
index 6142795b4..966e43f08 100755
--- a/src/librssguard/core/messagefilter.cpp
+++ b/src/librssguard/core/messagefilter.cpp
@@ -9,18 +9,52 @@
 MessageFilter::MessageFilter(int id, QObject* parent) : QObject(parent), m_id(id) {}
 
 FilteringAction MessageFilter::filterMessage(QJSEngine* engine) {
+  // NOTE: Filter is represented by JavaScript code, each filter must define
+  // function with "filterMessage()" prototype. There is a global "msg" object
+  // representing "message" available.
+  //
+  // All standard classes/functions as specified by ECMA-262 are available.
+  //
+  // MessageObject "msg" global object has some writable properties such as "title" or "author",
+  // see core/message.h file for more info.
+  //
+  // Note that function "filterMessage() must return integer values corresponding
+  // to enumeration "FilteringAction" (see file core/message.h).
+  // Also, there is a method MessageObject.isDuplicateWithAttribute(int) which is callable
+  // with "msg" variable and this method checks if given message already exists in
+  // RSS Guard's database. Method is parameterized and the parameter is integer representation
+  // of DuplicationAttributeCheck enumeration (see file core/message.h).
+  //
+  // Example filtering script might look like this:
+
   /*
-   * "(function() { "
+      function helper() {
+        if (msg.title.includes("A")) {
+          msg.isImportant = true;
+        }
 
-                                          //"return msg.isDuplicateWithAttribute(4) ? 1 : 2; "
-                                          "msg.isImportant = true;"
-                                          "return 1;"
-                                          "})"*/
+        return 1;
+      }
+
+      function filterMessage() {
+        return helper();
+      }
+   */
   QJSValue filter_func = engine->evaluate(m_script);
-  auto filter_output = filter_func.call().toInt();
-  FilteringAction decision = FilteringAction(filter_output);
 
-  return decision;
+  if (filter_func.isError()) {
+    qCritical("Error when evaluating script from filter '%d'. Error is: '%s'", id(), qPrintable(filter_func.toString()));
+    return FilteringAction::Accept;
+  }
+
+  auto filter_output = engine->evaluate(QSL("filterMessage()"));
+
+  if (filter_output.isError()) {
+    qCritical("Error when calling filtering function '%d'. Error is: '%s'", id(), qPrintable(filter_output.toString()));
+    return FilteringAction::Accept;
+  }
+
+  return FilteringAction(filter_output.toInt());
 }
 
 int MessageFilter::id() const {
diff --git a/src/librssguard/definitions/definitions.h b/src/librssguard/definitions/definitions.h
index 47d86c556..e05c8a557 100755
--- a/src/librssguard/definitions/definitions.h
+++ b/src/librssguard/definitions/definitions.h
@@ -115,7 +115,7 @@
 #define APP_DB_SQLITE_FILE            "database.db"
 
 // Keep this in sync with schema versions declared in SQL initialization code.
-#define APP_DB_SCHEMA_VERSION         "14"
+#define APP_DB_SCHEMA_VERSION         "15"
 #define APP_DB_UPDATE_FILE_PATTERN    "db_update_%1_%2_%3.sql"
 #define APP_DB_COMMENT_SPLIT          "-- !\n"
 #define APP_DB_NAME_PLACEHOLDER       "##"
diff --git a/src/librssguard/miscellaneous/databasequeries.cpp b/src/librssguard/miscellaneous/databasequeries.cpp
index 60dd38950..8eb6f081e 100755
--- a/src/librssguard/miscellaneous/databasequeries.cpp
+++ b/src/librssguard/miscellaneous/databasequeries.cpp
@@ -1428,9 +1428,32 @@ QList<MessageFilter*> DatabaseQueries::getMessageFilters(const QSqlDatabase& db,
 }
 
 QMultiMap<QString, int> DatabaseQueries::messageFiltersInFeeds(const QSqlDatabase& db, int account_id, bool* ok) {
-  // TODO: return list of relations
+  QSqlQuery q(db);
+  QMultiMap<QString, int> filters_in_feeds;
 
-  return {};
+  q.prepare("SELECT filter, feed_custom_id FROM MessageFiltersInFeeds WHERE account_id = :account_id;");
+
+  q.bindValue(QSL(":account_id"), account_id);
+  q.setForwardOnly(true);
+
+  if (q.exec()) {
+    while (q.next()) {
+      auto rec = q.record();
+
+      filters_in_feeds.insert(rec.value(1).toString(), rec.value(0).toInt());
+    }
+
+    if (ok != nullptr) {
+      *ok = true;
+    }
+  }
+  else {
+    if (ok != nullptr) {
+      *ok = false;
+    }
+  }
+
+  return filters_in_feeds;
 }
 
 QList<ServiceRoot*> DatabaseQueries::getStandardAccounts(const QSqlDatabase& db, bool* ok) {
diff --git a/src/librssguard/services/gmail/network/gmailnetworkfactory.cpp b/src/librssguard/services/gmail/network/gmailnetworkfactory.cpp
index 9fb008440..bbaef5d24 100644
--- a/src/librssguard/services/gmail/network/gmailnetworkfactory.cpp
+++ b/src/librssguard/services/gmail/network/gmailnetworkfactory.cpp
@@ -433,7 +433,8 @@ bool GmailNetworkFactory::obtainAndDecodeFullMessages(const QList<Message>& lite
   }
 }
 
-QList<Message> GmailNetworkFactory::decodeLiteMessages(const QString& messages_json_data, const QString& stream_id,
+QList<Message> GmailNetworkFactory::decodeLiteMessages(const QString& messages_json_data,
+                                                       const QString& stream_id,
                                                        QString& next_page_token) {
   QList<Message> messages;
   QJsonObject top_object = QJsonDocument::fromJson(messages_json_data.toUtf8()).object();