I never noticed something like this before... I guess flight numbers are another one of those mysteries of air travel (like how fares are figured out), but I've never noticed a flight between two points having the same flight number in both directions. I've seen the same flight number used when you have to change planes, or as a continuation on the same plane, but never as (effectively) a round trip.

SFO-RNO / RNO-SFO has UX 6438 (and at least one other that does the same thing). Doesn't that tend to screw things up (like boarding passes and baggage)?