Dear hoping,
I have never heard that individuals with a single kidney, from birth, accident, or donation have a higher incidence of kidney cancer than the rest of the population.
Major risk factors for kidney cancer are smoking and obesity. Smoking and obesity will exclude a prospective donor. It would be a bad idea for a donor to smoke or become obese post-donation.
Both donor kidneys are scanned prior to donation, hopefully any existing tumor would be found.
Therefore, kidney donors probably would have a lower rate of kidney cancer than the average.
Under the current allocation systems, donors have a higher priority than non-donors for kidney transplants.
Love, elephant