To add thickness, can a hair transplant be done twice on one place?

You can have multiple hair transplant surgeries, but you need to coordinate your plan with your doctor.

you may have a second surgery to increase the density , not the thickness of the hair. During the placement of follicular units there will be a mixture of single, double and a few triple grafts, which help to create the soft look of a natural hairline. 

Yes, it is possible to revisit a Hair Transplant and add additional density later on - basically implanting new follicles between the previous grafts - although you should only look to do this circa 12 months after the initial surgery.

Yes, Adding thickness to previous hair transplants are reasonable. Assuming that you had good growth but need more density, a second transplant is a good idea