Pretty much ALL literature over hundred and forty years had it spelled 'correctly' with one i
I've been curious to see the original description.
It doesn't really matter how many different publication mentioned it as
Dynastes granti with only one
i simply because two
ii is the very first name ever being published for this species (First come, first served goes with ICZN codes). Even if the original author misspelled, made a typo, or did not know the ICZN rules properly and named the new species with completely wrong standards, once it is published, that is the valid name. Unless the species name has different gender ending compares to the accompanying genus name ending, its ending cannot be changed.
If there is any publication with only one i, then the author simply did not know the correct name. Maybe they didn't check the original description, did not know there is a name with two ii, or didn't know about the ICZN codes, I guess...
By the way, in taxonomic research on this species, I believe
granti with only one
i ending never been published before, so all the authors who wrote the species name with only one i in their publication is simply a "mistake." It's not even a synonym. but just plain mistake.
One example where there is only one species with two well known names is:
Chalcosoma caucasus or
Chalcosoma chiron.
Until few years back, the species has been known as the
C. caucasus described by Fabricius in 1801. The name derived from the Greek term, meaning white snow, which probably been named after beetle's elytra being very reflective and shiny. This species, however, been named previously by Olivier in 1789 basing the specimens collected from the Java, Indonesia. Frank-Thorsten Krell in 2002, confirmed those two are actually one same species. Per the ICZN, the first name gets the validity of name, the
C. caucasus is synonymized, and
C. chiron became the valid name of the species.
Do you have the original description? The bugguide link leads to other writings.
https://www.jstor.org/stable/25076237?seq=10
Original description says:
"A specimen in my cabinet from Fort Grant, Arizona, has the thoracic horn very nearly twice as long as in our eastern specimens, the tip is broader and deeply emarginate, and the two small horns usually seen below the base of the larger are here reduced to small tubercles and are placed on the base of the horn itself. The frontal horn is also proportionately longer, distinctly grooved on its upper edge and with a tooth about one fourth from the tip limiting the groove in front. From the vase of thorax to tip of thoracic horn the length is 1.30 inch, in our eastern form a similar measurement gives 0.86 inch. The specimens have otherwise similar size and appearance. For this variety the name of Dynastes Grantii is proposed."
You can find this from the JSTOR link below, on page 78:
https://www.jstor.org/stable/25076237?seq=10
