Twice over
Definition 1 of 2
Pro Tip 1/3Repetition Emphasis
Using 'twice over' to show something is repeated for clarity or importance. 
She confirmed her appointment twice over, leaving no room for misunderstanding.
Pro Tip 2/3Reflective Assurance
Indicates careful reflection or consideration has been applied. 
He reviewed the contract twice over, ensuring every detail was considered.
Pro Tip 3/3Intensive Study
Implies an intensive, thorough study or examination of something. 
I went over the exam questions twice over to fully grasp the material.