I have a customer who has a house built in the 70's that has a 20 GFI breaker that feeds a lot of stuff from the house then a lot of lights down at the lake he lives next to (Jecuzzi, path lights, boat house lights etc.). He's wanting me to add an outlet down near the lake and installing a GFI in it also. Any problems adding it on the same circuit as the breaker?? I think I've run into this before and had no problems, but can't remember. Thanks Steve..
Its all down to trip times and device sensitivity. In a fault condition its a race between the devices and you can't predict which will go first. Hence the fault area is clear if only the one nearest the load trips, but a trip up the line and it's anyone's guess. In my opinion only time delayed units should be upstream of any others.
It's harder to trouble shoot per say because is is no the first thing that comes to your mind. I had once a GFCI breaker, GFCI recept and a second GFCI recept on the load side on the first recept. All is needed is one GFCI device and I place it in the most convient location in the circuit. If more then one is needed for conscience, no need to put one on the load side of another. If the customer don't know where they are all at then its gets time consuming meaning a big repair bill needlessly. Not good for a reputation.