it involves the use of ultrasound to assess flow-it's called "duplex" cuz it has a Doppler "line of sight" that is range gated so that flow in a particular vascular structure can be quantitated using analysis of the frequency spectrum-it is a safe procedure-without knowing more specifics, this is the best explanation i can offer

Duplex ultrasound includes doppler evaluation of the blood vessels. Specifically, the ovarian blood vessels in pelvic ultrasound or fetal blood vessels in the umbilical cord for obstetric ultrasound.

Duplex add Doppler blood flow info. Useful to look for possible IUGR or other vascular problems. No other significant difference.