diff --git a/app/scripts/app.js b/app/scripts/app.js
index 7a59a689..3caa72de 100644
--- a/app/scripts/app.js
+++ b/app/scripts/app.js
@@ -5,6 +5,7 @@ var AppModel = require('./models/app-model'),
     KeyHandler = require('./comp/key-handler'),
     IdleTracker = require('./comp/idle-tracker'),
     PopupNotifier = require('./comp/popup-notifier'),
+    SingleInstanceChecker = require('./comp/single-instance-checker'),
     Alerts = require('./comp/alerts'),
     Updater = require('./comp/updater'),
     AuthReceiver = require('./comp/auth-receiver'),
@@ -78,6 +79,7 @@ $(() => {
     function showView() {
         new AppView({ model: appModel }).render();
         Updater.init();
+        SingleInstanceChecker.init();
     }
 
     function getConfigParam() {
diff --git a/app/scripts/comp/single-instance-checker.js b/app/scripts/comp/single-instance-checker.js
new file mode 100644
index 00000000..f72a210c
--- /dev/null
+++ b/app/scripts/comp/single-instance-checker.js
@@ -0,0 +1,40 @@
+'use strict';
+
+const Backbone = require('backbone');
+const Launcher = require('./launcher');
+
+const LocalStorageKeyName = 'instanceCheck';
+const LocalStorageResponseKeyName = 'instanceMaster';
+
+let instanceKey = Date.now().toString();
+
+let SingleInstanceChecker = {
+    init: function() {
+        if (Launcher) {
+            return;
+        }
+        window.addEventListener('storage', SingleInstanceChecker.storageChanged);
+        SingleInstanceChecker.setKey(LocalStorageKeyName, instanceKey);
+    },
+
+    storageChanged: function(e) {
+        if (!e.newValue) {
+            return;
+        }
+        if (e.key === LocalStorageKeyName && e.newValue !== instanceKey) {
+            SingleInstanceChecker.setKey(LocalStorageResponseKeyName, instanceKey + Math.random().toString());
+        } else if (e.key === LocalStorageResponseKeyName && e.newValue.indexOf(instanceKey) < 0) {
+            window.removeEventListener('storage', SingleInstanceChecker.storageChanged);
+            Backbone.trigger('second-instance');
+        }
+    },
+
+    setKey: function(key, value) {
+        try {
+            localStorage.setItem(key, value);
+            setTimeout(() => { localStorage.removeItem(key); }, 100);
+        } catch (e) {}
+    }
+};
+
+module.exports = SingleInstanceChecker;
diff --git a/app/scripts/locales/base.json b/app/scripts/locales/base.json
index e454bfce..5d6aff1d 100644
--- a/app/scripts/locales/base.json
+++ b/app/scripts/locales/base.json
@@ -285,6 +285,8 @@
   "appSaveErrorBodyMul": "Failed to auto-save files:",
   "appSettingsError": "Error loading app",
   "appSettingsErrorBody": "There was an error loading app settings. Please double check app url or contact your administrator.",
+  "appTabWarn": "Too many tabs",
+  "appTabWarnBody": "KeeWeb cannot be used in two browser tabs simultaneously, please close this tab.",
 
   "setGenTitle": "General Settings",
   "setGenUpdate": "Update",
diff --git a/app/scripts/views/app-view.js b/app/scripts/views/app-view.js
index 5c4ca3e8..90a7c480 100644
--- a/app/scripts/views/app-view.js
+++ b/app/scripts/views/app-view.js
@@ -77,6 +77,7 @@ var AppView = Backbone.View.extend({
         this.listenTo(Backbone, 'user-idle', this.userIdle);
         this.listenTo(Backbone, 'app-minimized', this.appMinimized);
         this.listenTo(Backbone, 'show-context-menu', this.showContextMenu);
+        this.listenTo(Backbone, 'second-instance', this.showSingleInstanceAlert);
 
         this.listenTo(UpdateModel.instance, 'change:updateReady', this.updateApp);
 
@@ -617,6 +618,14 @@ var AppView = Backbone.View.extend({
         Backbone.trigger('context-menu-select', e);
     },
 
+    showSingleInstanceAlert: function() {
+        this.hideOpenFile();
+        Alerts.error({
+            header: Locale.appTabWarn, body: Locale.appTabWarnBody,
+            esc: false, enter: false, click: false, buttons: []
+        });
+    },
+
     dragover: function(e) {
         e.preventDefault();
     },
diff --git a/release-notes.md b/release-notes.md
index 0841b08b..076044f7 100644
--- a/release-notes.md
+++ b/release-notes.md
@@ -3,6 +3,7 @@ Release notes
 ##### v1.4.0 (WIP)
 `+` allow password copy on mobile Safari  
 `+` password generator usability improvements  
+`+` warning about several tabs  
 
 ##### v1.3.2 (2016-09-13)
 `-` fix #342: url detection in Microsoft Edge