Although it is rare to have more than one primary, it certainly is possible. If melanoma occurs within 2 centimeters of the original site of the tumor, it is called local recurrence. Typically, a second primary that is a local recurrence is thinner and more easily removed. The prognosis, if caught early, is actually very good. It can be better for a second, local recurrence than for a first primary, oddly enough.
It sounds as though you are understandably upset. I know I would be. I, too, thought I had a mole re-growing at my melanoma scar site. Since this is a newly developed mole, on a previous site that contained a melanoma tumor, I would ask one of your doctors (Primary Care, Dermatologist, Surgeon - some one!) to remove it. It might be considered elective, but if you want it removed, you should have that option. Anything removed from your skin should be biopsied - but you already know that!
I know it is NOT easy, but I wouldn't start panicking yet. But I would have it removed as soon as possible.