I don't know the answer, but I've shot a turkey with a double beard before, and I haven't shot a ton of turkeys, so it must be relatively common. The second beard on mine was not as full as the one in you picture though. It was a lot smaller than the main beard on my bird.
I've shot quite a few turkeys over the last 20 years (probably 35-36) and I've never shot a double beard
now a buddy of mine has shot 4 or 5 now with less total birds shot
maybe it's an area/genetics thing maybe pure luck I don't know
Not sure how common it is but they are out there, so not rare. Maybe as often as a bearded hen? I shot one years ago with 4 beards. Each one got smaller and smaller. It think the smallest was 1 inch.
think of it like duck bands. Yes the more birds you shoot the better the odds of getting one yet there are some who shoot few and happen to hit it right.
I shot one with 3 beards a couple of years ago and that was my first. It’s hard to determine how rare it is just from your experiences. You would have to get total numbers of birds shot in Ontario vs how many had more than one beard to get a good estimate. I think an albino turkey is more rare. I’ve only seen one in my life time.
"Give a man a fish and he eats for a day, Teach a man to fish and he eats for the rest of his life"