Can a stent have stenosis if the US says it's patent but the velocities are abnormally high? Is catheter angiogram next step?

Yes. Yes, it is often the case that high velocities through a vessel is suggestive of a higher grade stenosis Within.. and narrowing in a stent does occur by overgrowth of tissue called intimal hyperplasia. Angiogram through a catheter placed within the blood vessel can usually give more exact detail. Although you didn’t state which artery was stented, the general concepts are often similar.