My son has 2 regular outlets the second leads to the first of 4 GFCI outlets in the kitchen. When he uses either regular outlet it trips the GFCI. We changed both outlets although there was no apparent defects or damage. It still trips the GFCI. We then changed the GFCI. Again it trips when either of the two regular outlets are used. He can use the GFCI (the one that trips) with no problems and the three after the one that trips are fine. Which makes me believe the problem lies before the GFCI outlets begin. It drives me nuts when I can figure things out. Anyhow here's a list of the order of the outlets.
Regular - Regular - GFCI( the one that trips) - Switch for above sink light - GFCI - GFCI
How GFCI is wired below:
Left side one 3 whites Right side 2 blacks Bottom ground
GFCI outlet wiring drawing ,,,,,,,,,,,