User task updated in sequentially

Hello,

startActivity → ServiceTask1 → ServiceTask2 → UserTask1 → UserTask2 → endActivity

two thread starts with same businessKey, lets say userID.

ThreadA commits its state to DB as UserTask1 and proceed further. However, this thread gets detached from UI and UI triggers another call.

ThreadB again goes via ST1 adn ST2, but what will happen when it tries to update its state as userTask1 again ? will it throw any exception ? if no, will it override the existing state of process instance ?