I know some people who know even before the wave breaks that it is going to double up on the inside. What do you want to look for in a wave if you want to know if it will double up?
Its just experience in observing the waves at individual breaks. All breaks are going to be a little different, and after sitting out there for swell after swell you notice the trends.
Most of the Time there is a slower smaller wave in front And ur wave kinda just overruns it. Or at least thats the case here in Jax
Love me some double ups When the wave begins to peak it looks like this (cross section view) before breaking ___^/\___^/\___^/\__ vs. a normal set ___/\___/\___/\___ 2 things to note: a main peak with a smaller bump in front of it and the wave itself is thicker front to back. Also, they often suck out more and appear fatter from the leading edge to the back of the wave versus the normal swells coming in. Paddle hard for the smaller first peak and the extra water contained back wave plus the exaggerated suck out in front of both will help push you into a wave that probably didn't look very big or catchable but ends up being faster and steeper.
