I dunno how rare it is, but you've perfectly described the volunteers
I've got out there this season. Here's the largest.


Haha, that is beautiful . Alas, mine opened its first flower, and it's
yellow - I was hoping for red or orange. I'm growing several different
types this year, plus a dozen or so volunteers, so I think I'll save
some seed and see what I get next year. I wonder if I could consistently
produce the giant hydra sunflower?