I can only remember it happening once. I purchased some beans from cBay, when it was up and running of course, from a basement/garage breeder that had some great genetics and great imagination. He used several plants in a number of his crosses, normally something was crossed with G13 or Rosetta Stone or White Russian, sometimes crosses among them were made. I believe it might have been a G13 X White Russian cross but late in flower I had two small buds that began to form on two leaves.
At the time I assumed that it was some seldom seen genetic trait of one of the strains that were crossed to eventually end up the cross I grew, meaning it could be a very seldom trait from any one of a number of famous past strains.
Later thinking about it I had to wonder what the possibility is that something else caused it. Something like some small amount of insect nibbling or a slightly cracked/broken leaf/stem connection from say watering or adjusting of plants, and if it might possibly have a reaction like topping, stimulating a hormone telling the plant to create another budsite there.
I tend to think it is a deep genetic trait that seldom is seen but if charted accurately would likely trace back to one or two strains, if two both of them likely of the same general type and likely from a very similar region as the other, if not from the same region as the other. But I do have to wonder if there is any validity to the other wonderment?