array_count_associated

return array_map(function ($c, $serializedTicket) {

            $ticket = unserialize($serializedTicket);

            return [
                'id' => $ticket->get('id'),
                'name' => $ticket->get('name'),
                'amt' => $this->toDollarFormat($ticket->get('price')->toCent()),
                'qty' => $c
            ];

        }, array_count_values(
            array_map('serialize', $bill->getTickets())
        ), array_keys(array_count_values(
            array_map('serialize', $bill->getTickets())
        )));