--- a/browser/components/loop/MozLoopPushHandler.jsm
+++ b/browser/components/loop/MozLoopPushHandler.jsm
@@ -104,30 +104,35 @@ let MozLoopPushHandler = {
     */
   register: function(channelID, onRegistered, onNotification) {
     if (!channelID || !onRegistered || !onNotification) {
       throw new Error("missing required parameter(s):"
                       + (channelID ? "" : " channelID")
                       + (onRegistered ? "" : " onRegistered")
                       + (onNotification ? "" : " onNotification"));
     }
-    // Only register new channels
-    if (!(channelID in this.channels)) {
-      this.channels[channelID] = {
-        onRegistered: onRegistered,
-        onNotification: onNotification
-      };
+
+    // If the channel is already registered, callback with an error immediately
+    // so we don't leave code hanging waiting for an onRegistered callback.
+    if (channelID in this.channels) {
+      onRegistered("error: channel already registered: " + channelID);
+      return;
+    }
 
-      // If registration is in progress, simply add to the work list.
-      // Else, re-start a registration cycle.
-      if (this._registrationID) {
-        this._channelsToRegister.push(channelID);
-      } else {
-        this._registerChannels();
-      }
+    this.channels[channelID] = {
+      onRegistered: onRegistered,
+      onNotification: onNotification
+    };
+
+    // If registration is in progress, simply add to the work list.
+    // Else, re-start a registration cycle.
+    if (this._registrationID) {
+      this._channelsToRegister.push(channelID);
+    } else {
+      this._registerChannels();
     }
   },
 
   /**
    * Listener method, handles the start of the websocket stream.
    * Sends a hello message to the server.
    *
    * @param {nsISupports} aContext Not used

--- a/browser/components/loop/test/xpcshell/test_looppush_initialize.js
+++ b/browser/components/loop/test/xpcshell/test_looppush_initialize.js
@@ -41,18 +41,50 @@
         Assert.equal(mockWebSocket.protocol, "push-notification",
                      "Should have the protocol set to push-notifications");
         mockWebSocket.notify(15);
       },
       function(version, id) {
         Assert.equal(version, 15, "Should have version number 15");
         Assert.equal(id, "chan-1", "Should have channel id = chan-1");
         run_next_test();
+      });
+  });
+
+  add_test(function test_register_twice_same_channel() {
+    MozLoopPushHandler.register(
+      "chan-2",
+      function(err, url, id) {
+        Assert.equal(err, null, "Should return null for success");
+        Assert.equal(url, kEndPointUrl, "Should return push server application URL");
+        Assert.equal(id, "chan-2", "Should have channel id = chan-2");
+        Assert.equal(mockWebSocket.uri.prePath, kServerPushUrl,
+                     "Should have the url from preferences");
+        Assert.equal(mockWebSocket.origin, kServerPushUrl,
+                     "Should have the origin url from preferences");
+        Assert.equal(mockWebSocket.protocol, "push-notification",
+                     "Should have the protocol set to push-notifications");
+
+        // Register again for the same channel
+        MozLoopPushHandler.register(
+          "chan-2",
+          function(err, url, id) {
+            Assert.notEqual(err, null, "Should have returned an error");
+            // Notify the first registration to make sure that still works.
+            mockWebSocket.notify(16);
+          },
+          function(version, id) {
+            Assert.ok(false, "The 2nd onNotification callback shouldn't be called");
+        });
       },
-      mockWebSocket);
+      function(version, id) {
+        Assert.equal(version, 16, "Should have version number 16");
+        Assert.equal(id, "chan-2", "Should have channel id = chan-2");
+        run_next_test();
+      });
   });
 
   add_test(function test_reconnect_websocket() {
     MozLoopPushHandler.uaID = undefined;
     MozLoopPushHandler.registeredChannels = {}; //Do this to force a new registration callback.
     mockWebSocket.stop();
   });