Counting reps for one-legged (or one-armed) exercises?

I'm curious as to how people count reps for an exercise that uses only one leg (or one arm) at a time.

Say I do 10 eccentric heel drops on my left leg and then 10 on my right leg, and that I repeat that 3 times. Is that 3x10 or 3x20? I guess same question would apply for bicep curls or any other exercise that exercises one leg or one arm at a time.

I guess the logical answer is "as long as you're consistent and do it the same way each time, it doesn't really matter". But I'm still interested in hearing views.