How typical is a repeat ultra sound needed when scanning for a blood clot, is this a bad sign?

Comon. In a patient who is at high risk for developing a blood clot due to injury, surgery or genetic reasons it is not unreasonable to do repeated ultrasounds of the legs to look for clot. They are not dangerous and can be repeated as needed throughout your life.
Not necessarily. Ultrasounds can sometimes be technically challenging. If there is a continued clinical suspicion, there is nothing wrong with repeating the test. Better safe than sorry!