Imaged in Spain by Joaquin Querol. For a short while the bow split into two - not to be confused with supernumeraries. There were two storm systems and it is likely that the each bow came from a different rain cloud.
Image ©2019 Joaquin Querol, shown with permission
Twinned bows are formed when there are two distinctly different raindrop sizes. Either from two different rain clouds behind each other or mixed together. Small ~1mm dia. drops are spherical and give a nice circular bow. Larger drops are flattened by air resistance as they fall and generate slightly oval rainbows.
At right: A ray tracing made by replacing the mathematical ice crystals in HaloSim by raindrops.
A twinned bow is where the primary rainbow splits into two. The outer secondary bow is almost always undisturbed. Watch a rainbow closely because twinning is fugitive. It can flicker into sight for just a second or so.