Can the donor hair be reused in another transplant?

That's an interesting question.  It is possible. But generally transplanted hair is not replanted.

Once the hair follicle is taken from the donor area, it will not grow back in the same spot.  You may take other hair follicles from around the same area.

It is possible to use the same donor site for successive transplants if you have the appropriate density and your surgeon agrees with it. 

Yes, it depends of your donor availability after the 1st surgery. With good donor harvesting technique (strip and/or FUE) 1-2 or more surgeries can be performed.

The number of transplants possible depends on the available donor supply. With Strip FUT this is determined by the scar quality from previous surgery and the scalp laxity. With FUE this is determined by residual hair density and length. 

A surgeon can make multiple passes at the donor area until a point where the supply gets depleted.