-      my $invoice = $delivery_order->convert_to_invoice() || die $db->error;
-      push @invoices, $invoice;
+      # Only process open delivery orders. In this list should only be open
+      # delivery orders, but if the user clicked browser back, a new creation
+      # of invoices for delivery orders which are closed now can be triggered.
+      # Prevent this.
+      if ($delivery_order->closed) {
+        push @already_closed_delivery_orders, $delivery_order;
+
+      } else {
+        my $invoice = $delivery_order->convert_to_invoice() || die $db->error;
+
+        ARAP->close_orders_if_billed('dbh'     => $dbh,
+                                     'arap_id' => $invoice->id,
+                                     'table'   => 'ar',);
+
+        push @invoices, $invoice;
+      }