diff --git a/containers/libreportal/frontend/core/forms/js/custom-select.js b/containers/libreportal/frontend/core/forms/js/custom-select.js
index 972c0f3..c678068 100644
--- a/containers/libreportal/frontend/core/forms/js/custom-select.js
+++ b/containers/libreportal/frontend/core/forms/js/custom-select.js
@@ -23,7 +23,15 @@
 
   class CustomSelect {
     constructor(selectEl) {
-      if (selectEl[ENHANCED]) return;
+      // Guard against double-wrapping. The ENHANCED symbol is per-element, but a
+      // re-render race (the app config form re-renders on every tab switch) could
+      // slip a second enhancement through and nest a .custom-select inside another
+      // — showing the dropdown twice, one atop the other. The class + wrapper
+      // checks below survive that (they live on the DOM, not a JS symbol), so a
+      // select that's already enhanced or already inside a wrapper is never wrapped again.
+      if (selectEl[ENHANCED]
+          || selectEl.classList.contains('custom-select-native')
+          || selectEl.closest('.custom-select')) return;
       selectEl[ENHANCED] = true;
       this.select = selectEl;
       this.build();
@@ -300,6 +308,10 @@
   function shouldEnhance(el) {
     if (!(el instanceof HTMLSelectElement)) return false;
     if (el[ENHANCED]) return false;
+    // Already enhanced or already inside a wrapper — never nest a second widget
+    // (see the constructor). DOM-based so it survives a lost ENHANCED symbol.
+    if (el.classList.contains('custom-select-native')) return false;
+    if (el.closest('.custom-select')) return false;
     if (el.multiple) return false;                 // multi-selects need different UX
     if (el.hasAttribute('data-no-enhance')) return false;
     return ENHANCE_CLASSES.some(c => el.classList.contains(c));