Answer: Be split between Duke and the law firm, but how it will be split cannot be determined without more information.
Explanation: The law firm would have entered into a certain type of contract when signing Duke. Therefore the contract terms would determine how he is paid and also the bonus he would tend to get in times of outstanding performances from Duke the employee.
It's only when the contract terms is known that we can able to determine how much Duke would take home in a situation of an excess profit.