adjectivesfigures-of-speechis-it-a-wordnouns
I know one says an act of desperation, but I've heard desperacy much more than I've ever heard desperation, it's like I've almost never heard desperation.
Why exactly was desperation preferred over desperacy and is there a pattern to noun adjectives ending with -ate?
We would normally call an expression with a redundant adjective that served only to express a tautology, a pleonasm. Pleonasms are not always a bad thing, as they can add clarity or emphasis.
Note though:
A model is anyone whose body is used aesthetically, as a photographic or artistic subject, or as a background for clothes, demonstrably, or otherwise, and not necessarily pretty.
A djinn or spirit—for which genius is a synonym—may not be smart, and some quite stupid genii figure in some stories.
That monsters by several definitions, need not be dangerous.
That fast is relative, and some jets are much slower than others.
That some buoys can sink when damaged.
Hence these are not true pleonasms.
It is most likely that "abolition" is the more common form due in large part to its association with the "abolition movement".
France was one of the earlier countries to abolish slavery within its borders, and Société des amis des Noirs was one active group in the movement in France.
The picture on that web page refers to (pardon my French) L'Assemblée Nationale pour L'Abolition de la Traite des Noirs and is dated 1790. Clearly "abolition" was the preferred term in France, and that preference would have transferred to England.
Further, those advocating abolition in England and the US were "abolitionists". It would have been a mouthfull to call them "abolishmentists".
The main reason one might choose "abolishment" over "abolition" (aside from the subtle difference in meaning) is that "abolition" is associated with the movement to end slavery, and thus might be confusing or distracting when used to refer to a different process. But this confusion is not likely in native English speakers, unless the context contributes to the confusion.
Best Answer
I wonder where the OP has heard desperacy much more than desperation.
The OED says:
which suggest that it was a used word, about two centuries ago. Ngram can find no example, nor can Google Books, nor can COCA.
It is not that that desperacy is not an English word. It is not a current English word.