I just found "Lenses in Photography", Rudolf Kingslake, 1951 in a good used bookstore today. The optical block diagrams given in the book make it easy to see how a "duplet" is on a different level than a "doublet". Kingslake defines a duplet as "two components with a central stop, often symmetrical". A doublet refers to a cemented lens pair. A duplet can be a pair of single-element lenses, doublets, or triplets. A singlet can be composed of a single element lens or a cemented group. It's more than just how many elements and groups are in an optic, but more describes their layout.
Kingslake also writes about Aspheric Optics, and the manufacturing challenges that need to be overcome to produce them. Great book from a Brilliant mind.