Two players on the same team can have the same number, but cannot be on the field at the same time. There are usually 100+ players on a college team's roster. The overlap of a number usually occurs between seniors and freshmen, or with second- or third-string players who will see little to no game time.
No they can not, for the simple reason of identifying players from the referee's stanpoint. If you had two players with the same number, calling a penalty on the player could mean two different players. So unfortunately, you'll just have to ask another player for the number if it's taken.
Since penalties are not tallied by player and used to inact a subsequent penalty, ie fouls in basketball and sequentially fouling out, the use of the same number by multiple players is allowed in football and other sports without aggregate penalty systems.