diff --git a/php/public/second-tab-warning.js b/php/public/second-tab-warning.js
new file mode 100644
index 00000000..63abc18b
--- /dev/null
+++ b/php/public/second-tab-warning.js
@@ -0,0 +1,12 @@
+const channel = new BroadcastChannel('tab')
+
+channel.postMessage('second-tab')
+// note that listener is added after posting the message
+
+channel.addEventListener('message', (msg) => {
+    if (msg.data === 'second-tab') {
+        // message received from 2nd tab
+        document.getElementById('overlay').classList.add('loading')
+        alert('Cannot open multiple instances. You can use AIO here by reloading the page.')
+    }
+});
\ No newline at end of file
diff --git a/php/templates/containers.twig b/php/templates/containers.twig
index e3851142..d698abb9 100644
--- a/php/templates/containers.twig
+++ b/php/templates/containers.twig
@@ -18,6 +18,9 @@
     <div class="content">
         <h1>Nextcloud AIO v2.0.4</h1>
 
+        {# Add 2nd tab warning #}
+        <script type="text/javascript" src="second-tab-warning.js"></script>
+
         {% set isAnyRunning = false %}
         {% set isAnyRestarting = false %}
         {% set isWatchtowerRunning = false %}