diff --git a/pkg/checkout/mutation_initialize_checkout.go b/pkg/checkout/mutation_initialize_checkout.go
index ec58f94..bfe5c83 100644
--- a/pkg/checkout/mutation_initialize_checkout.go
+++ b/pkg/checkout/mutation_initialize_checkout.go
@@ -4,6 +4,7 @@ import (
 	"encoding/json"
 	"fmt"
 
+	"git.k6n.net/go-cart-actor/pkg/cart"
 	messages "git.k6n.net/go-cart-actor/proto/checkout"
 )
 
@@ -32,7 +33,12 @@ func HandleInitializeCheckout(g *CheckoutGrain, m *messages.InitializeCheckout)
 	// 	return fmt.Errorf("InitializeCheckout: missing orderId")
 	// }
 	if g.CartState != nil {
-		return fmt.Errorf("InitializeCheckout: checkout already initialized")
+		if g.CartId != cart.CartId(m.CartId) {
+			return fmt.Errorf("InitializeCheckout: cart ID mismatch")
+		}
+		if g.PaymentInProgress > 0 || g.OrderId != nil {
+			return fmt.Errorf("InitializeCheckout: payment already in progress")
+		}
 	}
 
 	err := json.Unmarshal(m.CartState.Value, &g.CartState)