diff --git a/app/sql.js b/app/sql.js
index 623f487f1..8c24d3e2a 100644
--- a/app/sql.js
+++ b/app/sql.js
@@ -89,6 +89,9 @@ module.exports = {
 
   getMessageCount,
   saveMessage,
+  cleanSeenMessages,
+  saveSeenMessageHashes,
+  saveSeenMessageHash,
   saveMessages,
   removeMessage,
   getUnreadByConversation,
@@ -98,6 +101,7 @@ module.exports = {
   getAllMessageIds,
   getAllUnsentMessages,
   getMessagesBySentAt,
+  getSeenMessagesByHashList,
   getExpiredMessages,
   getOutgoingWithoutExpiresAt,
   getNextExpiringMessage,
@@ -390,6 +394,13 @@ async function updateToSchemaVersion6(currentVersion, instance) {
   console.log('updateToSchemaVersion6: starting...');
   await instance.run('BEGIN TRANSACTION;');
 
+  await instance.run(
+    `CREATE TABLE seenMessages(
+      hash STRING PRIMARY KEY,
+      expiresAt INTEGER
+    );`
+  );
+
   // key-value, ids are strings, one extra column
   await instance.run(
     `CREATE TABLE sessions(
@@ -1230,6 +1241,45 @@ async function saveMessage(data, { forceSave } = {}) {
   return toCreate.id;
 }
 
+async function saveSeenMessageHashes(arrayOfHashes) {
+  let promise;
+
+  db.serialize(() => {
+    promise = Promise.all([
+      db.run('BEGIN TRANSACTION;'),
+      ...map(arrayOfHashes, hashData => saveSeenMessageHash(hashData)),
+      db.run('COMMIT TRANSACTION;'),
+    ]);
+  });
+
+  await promise;
+}
+
+async function saveSeenMessageHash(data) {
+  const {
+    expiresAt,
+    hash,
+  } = data;
+  await db.run(
+    `INSERT INTO seenMessages (
+      expiresAt,
+      hash
+    ) values (
+      $expiresAt,
+      $hash
+    );`, {
+      $expiresAt: expiresAt,
+      $hash: hash,
+    }
+  );
+}
+
+async function cleanSeenMessages() {
+  await db.run('DELETE FROM seenMessages WHERE expiresAt <= $now;', {
+    $now: Date.now(),
+  });
+}
+
 async function saveMessages(arrayOfMessages, { forceSave } = {}) {
   let promise;
 
@@ -1360,6 +1410,15 @@ async function getMessagesBySentAt(sentAt) {
   return map(rows, row => jsonToObject(row.json));
 }
 
+async function getSeenMessagesByHashList(hashes) {
+  const rows = await db.all(
+    `SELECT * FROM seenMessages WHERE hash IN ( ${hashes.map(() => '?').join(', ')} );`,
+     hashes
+  );
+
+  return map(rows, row => row.hash);
+}
+
 async function getExpiredMessages() {
   const now = Date.now();
 
diff --git a/js/background.js b/js/background.js
index 07bcbcde6..5e7563c6b 100644
--- a/js/background.js
+++ b/js/background.js
@@ -464,7 +464,13 @@
     }
   });
 
+  function manageSeenMessages() {
+    window.Signal.Data.cleanSeenMessages();
+    setTimeout(manageSeenMessages, 1000 * 60 * 60);
+  }
+
   async function start() {
+    manageSeenMessages();
     window.dispatchEvent(new Event('storage_ready'));
 
     window.log.info('listening for registration events');
diff --git a/js/modules/data.js b/js/modules/data.js
index 0123eafe4..a31ed1c74 100644
--- a/js/modules/data.js
+++ b/js/modules/data.js
@@ -122,6 +122,9 @@ module.exports = {
 
   getMessageCount,
   saveMessage,
+  cleanSeenMessages,
+  saveSeenMessageHash,
+  saveSeenMessageHashes,
   saveLegacyMessage,
   saveMessages,
   removeMessage,
@@ -140,6 +143,7 @@ module.exports = {
   getOutgoingWithoutExpiresAt,
   getNextExpiringMessage,
   getMessagesByConversation,
+  getSeenMessagesByHashList,
 
   getUnprocessedCount,
   getAllUnprocessed,
@@ -728,6 +732,18 @@ async function getMessageCount() {
   return channels.getMessageCount();
 }
 
+async function cleanSeenMessages() {
+  await channels.cleanSeenMessages();
+}
+
+async function saveSeenMessageHashes(data) {
+  await channels.saveSeenMessageHashes(_cleanData(data));
+}
+
+async function saveSeenMessageHash(data) {
+  await channels.saveSeenMessageHash(_cleanData(data));
+}
+
 async function saveMessage(data, { forceSave, Message } = {}) {
   const updated = keysFromArrayBuffer(MESSAGE_PRE_KEYS, data);
   const id = await channels.saveMessage(_cleanData(updated), { forceSave });
@@ -861,6 +877,13 @@ async function getMessagesByConversation(
   return new MessageCollection(encoded);
 }
 
+async function getSeenMessagesByHashList(
+  hashes
+) {
+  const seenMessages = await channels.getSeenMessagesByHashList(hashes);
+  return seenMessages;
+}
+
 async function removeAllMessagesInConversation(
   conversationId,
   { MessageCollection }
diff --git a/libtextsecure/http-resources.js b/libtextsecure/http-resources.js
index bb1fa9ee0..576ae98d9 100644
--- a/libtextsecure/http-resources.js
+++ b/libtextsecure/http-resources.js
@@ -40,6 +40,17 @@
     };
   };
 
+  const filterIncomingMessages = async function filterIncomingMessages(messages) {
+      const incomingHashes = messages.map(m => m.hash);
+      const dupHashes = await window.Signal.Data.getSeenMessagesByHashList(incomingHashes);
+      const newMessages = messages.filter(m => !dupHashes.includes(m.hash));
+      const newHashes = newMessages.map(m => ({
+        expiresAt: m.expiration,
+        hash: m.hash,
+      }));
+      await window.Signal.Data.saveSeenMessageHashes(newHashes);
+      return newMessages;
+  };
 
   window.HttpResource = function HttpResource(_server, opts = {}) {
     server = _server;
@@ -68,7 +79,8 @@
         setTimeout(() => { pollServer(callBack); }, 5000);
         return;
       }
-      result.messages.forEach(async message => {
+      const newMessages = await filterIncomingMessages(result.messages);
+      newMessages.forEach(async message => {
         const { data } = message;
         const dataPlaintext = stringToArrayBufferBase64(data);
         const messageBuf = textsecure.protobuf.WebSocketMessage.decode(dataPlaintext);