Allocating to Flights and Seeding

Hello: Lets say I want to randomly divide competitors into three
flights. How does one do it with SW? I thought that Seeding should
randomize competitors then it would be easy to allocate each
competitor to a flight in turn. But Numeric Random produces a weird
effect and some sail numbers appear twice. My numbers run from 1 to
18. What does seeding actually do?
Ideas on randomly allocating competitors to flights?
Thanks, Donald