diff --git a/app/konto/components/LogoutButton.tsx b/app/konto/components/LogoutButton.tsx
index ad5f88d..28985f5 100644
--- a/app/konto/components/LogoutButton.tsx
+++ b/app/konto/components/LogoutButton.tsx
@@ -2,12 +2,20 @@
 
 import { useRouter } from "next/navigation";
 import { dispatchAuthChanged } from "../../lib/auth";
+import { clearCart } from "../../lib/cart";
 
 export function LogoutButton() {
   const router = useRouter();
 
   async function handleLogout() {
     await fetch("/api/account/logout", { method: "POST" });
+    // The local cart is already mirrored server-side by CartSync, so it's
+    // safe to clear it here — the next login's mergeServerCartIntoLocal()
+    // restores it from the server. Without this, the local cart survived
+    // logout untouched, and mergeServerCartIntoLocal()'s additive merge
+    // (existing.qty += qty) would add the already-synced server quantities
+    // on top of it on every login, doubling every logout/login cycle.
+    clearCart();
     dispatchAuthChanged();
     router.push("/");
     router.refresh();
diff --git a/app/lib/cart.ts b/app/lib/cart.ts
index 519189b..18ffcce 100644
--- a/app/lib/cart.ts
+++ b/app/lib/cart.ts
@@ -18,7 +18,8 @@ function readCart(): CartItem[] {
   if (typeof window === "undefined") return [];
   try {
     const raw = window.localStorage.getItem(CART_KEY);
-    return raw ? JSON.parse(raw) : [];
+    const parsed = raw ? JSON.parse(raw) : [];
+    return Array.isArray(parsed) ? parsed : [];
   } catch {
     return [];
   }
@@ -90,7 +91,8 @@ export function getCart(): CartItem[] {
   if (raw === cachedRaw) return cachedItems;
   cachedRaw = raw;
   try {
-    cachedItems = raw ? JSON.parse(raw) : EMPTY_CART;
+    const parsed = raw ? JSON.parse(raw) : EMPTY_CART;
+    cachedItems = Array.isArray(parsed) ? parsed : EMPTY_CART;
   } catch {
     cachedItems = EMPTY_CART;
   }
@@ -125,9 +127,14 @@ export function useCart(): CartItem[] {
 // Called right after a successful login (LoginForm.tsx, CheckoutContent.tsx's
 // inline login toggle) — folds whatever was saved server-side into the
 // local cart by quantity (addToCart adds to an existing line rather than
-// overwriting it), so items added before logging in aren't lost. CartSync
-// then picks up the resulting change and pushes the merged cart back to
-// the server on its own, closing the loop without a separate save call here.
+// overwriting it), so items added as a guest before logging in aren't lost.
+// This only stays correct because LogoutButton.tsx clears the local cart on
+// logout — the local cart is always either empty (no guest additions since
+// the last logout) or holds only genuinely new guest-session items, never a
+// stale copy of what's already in the server cart, so this add never
+// double-counts. CartSync then picks up the resulting change and pushes the
+// merged cart back to the server on its own, closing the loop without a
+// separate save call here.
 export async function mergeServerCartIntoLocal(): Promise<void> {
   try {
     const res = await fetch("/api/account/cart");