Chrysanthemums and double orange blossom

With regard to the Chrysanthemum, it sounds like proliferation, a known phenomenon that most frequently affects roses, but also plants in the Asteraceae family of which Chrysanthemum is a memnber. It is considered a mutation, though the cause is not entirely known; it essentially means other blooms forming in an existing bloom on a plant. The phenomenon is usually restricted to only one flower, with subsequent flowers being unaffected. Further info here https://www.rhs.org.uk/advice/profile?pid=634.

In regard to your 'orange blossom', since its not entirely clear precisely what plant you're speaking of, other than to say it doesn't fall under proliferation by the sound of it, perhaps you could add a photo, give the botanical name of the plant, or clarify which plant you mean, as well as saying whether the double and single flowers occur on separate branches, or are mixed on the same branch.