diff --git a/js/background.js b/js/background.js
index c8e73c661..06064f418 100644
--- a/js/background.js
+++ b/js/background.js
@@ -200,6 +200,11 @@
         }
 
         if (ev.proto) {
+            if (e.name === 'MessageCounterError') {
+                // Ignore this message. It is likely a duplicate delivery
+                // because the server lost our ack the first time.
+                return;
+            }
             var envelope = ev.proto;
             var message = initIncomingMessage(envelope.source, envelope.timestamp.toNumber());
             message.saveErrors(e).then(function() {
diff --git a/js/libtextsecure.js b/js/libtextsecure.js
index 05fe834a1..fdc526b2b 100644
--- a/js/libtextsecure.js
+++ b/js/libtextsecure.js
@@ -34806,7 +34806,9 @@ window.axolotl.protocol = function(storage_interface) {
                 return fillMessageKeys(chain, message.counter).then(function() {
                     var messageKey = chain.messageKeys[message.counter];
                     if (messageKey === undefined) {
-                        throw new Error("Message key not found. The counter was repeated or the key was not filled.");
+                        var e = new Error("Message key not found. The counter was repeated or the key was not filled.");
+                        e.name = 'MessageCounterError';
+                        throw e;
                     }
                     return HKDF(axolotlInternal.utils.convertToArrayBuffer(messageKey), '', "WhisperMessageKeys").then(function(keys) {
                         return storage_interface.getMyIdentityKey().then(function(ourIdentityKey) {
diff --git a/libtextsecure/libaxolotl.js b/libtextsecure/libaxolotl.js
index 5b00cea1c..1f74c7bde 100644
--- a/libtextsecure/libaxolotl.js
+++ b/libtextsecure/libaxolotl.js
@@ -34708,7 +34708,9 @@ window.axolotl.protocol = function(storage_interface) {
                 return fillMessageKeys(chain, message.counter).then(function() {
                     var messageKey = chain.messageKeys[message.counter];
                     if (messageKey === undefined) {
-                        throw new Error("Message key not found. The counter was repeated or the key was not filled.");
+                        var e = new Error("Message key not found. The counter was repeated or the key was not filled.");
+                        e.name = 'MessageCounterError';
+                        throw e;
                     }
                     return HKDF(axolotlInternal.utils.convertToArrayBuffer(messageKey), '', "WhisperMessageKeys").then(function(keys) {
                         return storage_interface.getMyIdentityKey().then(function(ourIdentityKey) {