Hi @g.elund and welcome to the forum 😊 Great sunflower ... multiheaded ones aren’t really unusual ... there are quite a few varieties that produce several flower heads as well as different colours and petal formations.
I have the same sort of sunflower in my garden, we brought the seeds from France, to start with all the sunflowers that grew where single heads, this one appeared after all the rest had died of, its a beauty with many more heads on the stems with the large head as normal
@g.elund : What a delightful surprise from your sunflower! *I had the same thing happen a few years ago & ended up with a passable sunflower-coloured 'hedge' from just six plant stems! BUT I didn't manage to save any viable seed to repeat the 'accident' in following seasons - maybe you'll be luckier & can look forward to stageing a repetition of your success!! [? please can be cheeky & beg a handful of your saved seed to emulate your triumph??].
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https://www.mr-fothergills.co.uk/Flower-Seed/Sunflower-Seed/#.X2XybSR4XYU
Gardening in Central Norfolk on improved gritty moraine over chalk ... free-draining.