It really depends on how dense your donor hair is. A qualified physician may be able to harvest follicles from the same donor site, but it really depends on the density of your hair at this location. If your hair is too sparse, it may not be a good idea, because there won't be enough left over to cover the donor site. That said, you can select another area of your head as a donor site if the original site is no longer eligible. Please consult with a qualified surgeon to learn more about your options.
Once a hair follicle is removed from the terminal hair zone and transplanted to the front, sides, crown etc it is gone from the original donor area and will not regrow. When one removes the hair follicle in an FUE/FUT procedure one removes the entire follicle which includes the hair bulb/papilla, inner root sheath, outer root sheath, the hair shaft etc basically an entire follicular unit is removed and transplanted.
Once the hair follicle is harvested, it is gone from that spot. It will not regrow new hair. You can harvest from around the area.
When a hair follicle is removed, it is gone in that area. but the area has more hair follicles that can be moved provided that your doctor does not over-harvest the area.
Whether you do FUT/strip or FUE, there is a finite amount of hair you can move around for hair restoration. Once you remove hair follicles from the back and put them in the area of thinning, they do not grow back in the original site. Some people may be fortunate enough to have density and laxity that allows for 6,000-8000 grafts or more to harvest over many years or even decades. Other patients do not. Because the FUE procedure thins out the entire donor area more uniformly, it is unlikely to get more than 2 procedures through this method unless they are very small procedures because eventually can be come patchy from over-harvesting. When you remove a strip of scalp through FUT, you are more likely to be able to go back multiple times for additional procedures. However, this depends on charachteristics of your scalp and the density of your donor hair.
Most people have the ability to have more than one hair transplant surgery. On average, people have about 4000-8000 usable grafts available in the donor area. Those grafts, of course, cannot all be removed at the same time. The number of grafts available depends on a number of factors including, density of hair, size of head, and extent of loss over the years.
When grafts are removed via the strip method, a linear scar is made. When there is a subsequent surgery, more grafts are taken from the same area. The same is true for FUE. Any additional grafts are taken from the same area.