🪲 Prevent registering the same type twice

prefs.js

@@ -304,6 +304,7 @@ var PreferencesDialog = class PreferencesDialog {
     // Each effect page is based on a template widget. This template contains the title
     // and the preview button.
     // clang-format off
+    if (GObject.type_from_name('BurnMyWindowsEffectPage') == null) {
     BurnMyWindowsEffectPage =
       GObject.registerClass({
         GTypeName: 'BurnMyWindowsEffectPage',
@@ -373,6 +374,7 @@ var PreferencesDialog = class PreferencesDialog {
       // clang-format on
     }
   }
+}
 
 // Nothing to do for now...
 function init() {}