Multiple head marigolds

robin-bates

I have in my garden a marigold plant as pictured below. I'm new to gardening and didn't think anything of it, but a professional gardening friend  says she has not seen multiple headed flowers  like these before. Please let me know if this ls a rare occurrence and what might cause it.

fidgetbones

It is called hen and chickens marigold, aka Calendula officinalis prolifera