What is the ruling on repeating ablution and prayer due to excessive obsessive doubts concerning them?

There is no cure for obsessive whispers (waswas) except to disregard them and pay them no mind. So, when you perform ablution (wudu'), limit yourself to a single ablution and do not repeat it. Similarly, when you pray, strive not to pray more than once. Turn away from any illusion of defect in your purification or prayer.