The guidance on this in the NCCI manual is as follows:
Abdominal ultrasound examinations (CPT codes 76700-76775) and abdominal duplex examinations (CPT codes 93975, 93976) are generally performed for different clinical scenarios, although there are some instances where both types of procedures are medically reasonable and necessary . In the latter case, the abdominal ultrasound procedure CPT code should be reported with an NCCI PTP-associated modifier.
So as long as your documentation supports both the duplex studies and ultrasounds as distinct and medically necessary services, then your modifier 59 in these cases would be appropriate.
However, I don't think you can bill two duplex codes 93975 because the complete study includes the vascular imaging for both organs and there is a MUE limit of 1 unit per day for this code.