If the chance of rolling the same numbers is low (for example your range is 1-100, and you wanna fill 6 fields), I would probably just do a while loop (roll -> check if number is present; if it is, reroll; end loop when number is not present).
If the number of fields is closer to the range of values you are filling into them, I would do something like:
this would circumvent unnecessary retries, but need a bit more memory, since you build an array (but memory would only be a problem for big ranges)