Hi Jeff,
Exact same problem here on a 2002 Elantra with manual transmission. I changed the throttle position sensor with no help. I can normally avoid the engine revving to high by quickly releasing the clutch just after startup (I always startup the engine in neutral). My theory is that the engine is having an extra load when the clutch is released. This extra load slows down a little bit the rpm and, for some reasons, the computer will finally realize this is idle condition.
By the way, I also find another "anomaly" which is maybe related to overrevving at startup: when changing gears from 1st to 2nd or from 2nd to
3rd, the engine drops rpm slowly when warm. Does not happen when changing from 3rd to 4th or 4th to 5th or when the engine is warming up or when the engine is loaded with AC or rear defroster.