diff --git a/docs/NEWS b/docs/NEWS
index 9e200221..b8b4475c 100644
--- a/docs/NEWS
+++ b/docs/NEWS
@@ -4,6 +4,9 @@
 Version 2.0-beta3 ()
 ------------------------------------------------------------------------
 
+    * Prevent "new" plugin api to install double instances of plugins
+      that are not stackable (issue #45)
+
     * Back button in plugin-config
 
     * Adapted serendipity_editor.js to provide more global (though
diff --git a/include/admin/plugins.inc.php b/include/admin/plugins.inc.php
index 75dccee2..ae9066ae 100644
--- a/include/admin/plugins.inc.php
+++ b/include/admin/plugins.inc.php
@@ -345,7 +345,19 @@ if (isset($_GET['serendipity']['plugin_to_conf'])) {
                                   'install' => true);
         serendipity_plugin_api::hook_event('backend_plugins_fetchplugin', $fetchplugin_data);
 
-        if ($fetchplugin_data['install']) {
+        // we now have to check that the plugin is not already installed or stackable to prevent invalid double instances
+        $existingPlugin =& serendipity_plugin_api::load_plugin($serendipity['GET']['install_plugin']);
+        $new_plugin = true;
+        if (is_object($existingPlugin)) {
+            $bag = new serendipity_property_bag();
+            $existingPlugin->introspect($bag);
+            if ($bag->get('stackable') != true) {
+                $new_plugin = false;
+            }
+        }
+        $data['new_plugin_failed'] = ! $new_plugin;
+
+        if ($fetchplugin_data['install'] && $new_plugin) {
             $serendipity['debug']['pluginload'] = array();
             $inst = serendipity_plugin_api::create_plugin_instance($serendipity['GET']['install_plugin'], null, (serendipity_plugin_api::is_event_plugin($serendipity['GET']['install_plugin']) ? 'event': 'right'), $authorid, serendipity_db_escape_string($serendipity['GET']['pluginPath']));
 
diff --git a/templates/2k11/admin/plugins.inc.tpl b/templates/2k11/admin/plugins.inc.tpl
index 5f56554e..4649efa4 100644
--- a/templates/2k11/admin/plugins.inc.tpl
+++ b/templates/2k11/admin/plugins.inc.tpl
@@ -146,6 +146,9 @@
     {if $save}
     <span class="msg_success"><span class="icon-ok-circled"></span> {$CONST.DONE}:{$CONST.SETTINGS_SAVED_AT|sprintf:"$timestamp"}</span>
     {/if}
+    {if $new_plugin_failed}
+        <span class="msg_error"><span class="icon-attention-circled"></span> {$CONST.ERROR}: Plugin not stackable and already installed</span>  {* i18n *}
+    {/if}
     <script src="{serendipity_getFile file="admin/js/jquery.autoscroll.js"}"></script>
     <script src="{serendipity_getFile file="admin/js/jquery.sortable.js"}"></script>
     <script src="{serendipity_getFile file="admin/js/dragdrop.js"}"></script>