dadiOH wrote:
I'm working on our master bath, wife wanted a second duplex outlet near
the vanity. OK, a PITA but no problem.

My plan was to daisy chain off the existing GFI outlet, running romex
from it to the desired location. Problem is, the existing outlet has
two romex cables coming into it...one is standard 12/2 (plus ground) -
call it "A" - which is attached to one side of the outlet. The other is
12/4 romex (with two ground wires), call it "B". One pair of black &
white "B" wires go to the side of the duplex opposite "A"; they then are
pigtailed to the second pair of black & white "B" wires.

I'm guessing that romex "A" is what is feeding the existing outlet and
that the "B" wires go somewhere - I know not where - to feed whatever.

If I want the potential duplex to be GFI, I need to feed it from the
outgoing side of the existing outlet, correct? And how can I tell for
sure which romex - "A" or "B" is feeding the existing outlet?

Hi,
Can't you find a marking like LINE(your A side) and LOADB). Sounds like
that one is already daisy chained to some other outlet. If you want to
find which outlet is connected, trip it by pushing the test button,
and find out which outlet lost power.