As shown Erf-b2-p4, it appears that Dittomancy amplifies bonuses and duplicates units.
Jetstone's Dittomancer has a tendency to say everything twice. For example, "Yeah, yeah. I know, I know", or "It's... It's foolamancy! Foolamancy!". Where each word or phrase is said twice, in keeping with the doubling theme. He also appears to rhyme when precise repeats are not what he wishes to say. The interaction of Dittomancy with Rhymomancy is still unknown.
- Doubling leadership, special bonuses, and duplicating surviving units.
- Increasing the number of ranged attacks from a set of specified units, either double or quadruple (possibly more).
Given the name and the alignment (Stuff, numbers) of the class, dittomancy probably involves multiplying or copying stuff.
A dittomancer has been seen multiplying the number of arrows in an archer stack's volley. This may be a physical manifestation of multiplying the archers' combat stat. Dittomancers may also be able to double other stats, such as hits or move.
Dittomancers may be able to make copies of other units, in a fashion similar to croakamancy. Eg, the more attention spent on each individual unit, the more like the original it will be. It may be possible to duplicate an entire army, but the resulting copies would likely have very low stats and last only for a single turn.
Real World References
"Ditto" is a term that has many iterations, all of which involve duplication, repetition, or copying.
The original Tuscan word (cir 1625) meant "in the stated month or year" and was used in place of repeating the same date. By 1678 it was in use in English to mean "same as above," from which we get the "ditto mark" (or "), placed under a word or phrase it is used to repeat.
The Ditto corporation made simple printing devices called spirit duplicators (because they used alcohol), or "Ditto machines" which produced "ditto sheets," or copies.
Ditto was used famously in the film Ghost to mean, roughly, "I feel the same way."
Multiple fictional characters that involve copying or twins, etc. are named ditto.