Imladir:
This is exactly what you don't want: if you reroll like that, you effectively get the 1/13th chance to get anything, whereas you should have 8/20 chance to get an 8 and 1/20 for all the rest.
Oh yeah. We want to roll from deck of [8, 8, 8, 8, 8, 8, 8, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20].