γγ for κκ

Does γγ ever replace κκ in the way that Attic ττ becomes σσ in Ionic?
I’m trying to figure out the source of a word used in modern botanical nomenclature: coggygria. It’s very close to κοκκυγέα, a word used in the same sense as the modern word.
I’m also curious about how, if a word such as κογγυγρια existed, it would have been pronounced. In particular, if the γγ is derived from κκ, would the initial γ have been pronounced as a nasal γ? And if so, would a proper romanization be congygria?

No, not to my knowledge. -γγ- would be nasalized, but not –κκ-. It must be just a later misspelling of κοκκ-/cocc-, and the gg’s, like the cc’s/κκ’s, will have been hard (despite the fact that coccyx, as I discovered in hospital after being poleaxed and concussed by slipping on the ice on Boston Common one New Year’s Eve, is now pronounced with sibilant second c, cock-six). Presumably it’s phonetic in origin; γ(g) and κ(c) do sometimes get confused in spelling, being so close in sound.

EDIT. A quick google search leads to http://uses.plantnet-project.org/fr/Dictionnaire_étymologique, which tracks the history of the garbled form. The entry reads:
coggygria
épithète de Cotinus coggygria introduite par Scopoli (1771)
“commonly called coccygria” Miller (1754).
Cocconilea sive Coggygria Caspar Bauhin, Pinax theatri botanici, 1623, p. 415.
Coccigria sive Cotinus putata J. Bauhin I 4, 494. (à vérifier)
< latin classique coccygia :
coccygia… Proprietatem habet fructum amittendi lanugine (pappum uocant), quod nulli alii arbori euenit. (Pline, 13, 121).
[le coccygia… Il a la propriété de transformer son fruit en une masse cotonneuse, que l’on nomme pappus, ce que nul autre arbre ne fait.]
Le latin est emprunté au grec κοκκυγέα (ἡ) - kokkugea, de même sens chez Théophraste ou κοκκυγία - kokkugia.
Le nom grec a été rapproché de κόκκυξ - kokkux, (= coucou (l’oiseau)), mais la glose d’Hésychios κόκκυς· λόφος - kokkus : lophos (= aigrette) convient mieux, l’arbre à perruques étant alors “l’arbre à aigrettes”. Chez Hésychios, κοκκυγία - kokkugia est aussi le nom d’une anémone, qui a également un fruit plumeux.
Il peut sembler bizarre que les botanistes aient retenu une forme corrompue du latin classique. Bauhin cite des formes similaires de L’Obel, Clusius et Césalpin. Dans l’édition de Théophraste par Bodaeus a Stapel (1644), Scaliger cite Coggrygia de Pline, et Robert Constantin coggygria (p. 235). Ces graphies devaient donc se trouver dans les manuscrits de Pline disponibles à la Renaissance.

OED gives this pronunciation [ˈkɒksɪks]. But isn’t that what we’d expect, the common English pronunciation of -cc- before a front vowel? Eccentric occidental accident.

Eccentric occidental accident—yes, that’s what befell my (uncompounded) coccyx.

So, if I decipher the French correctly, the word was a graphical mistake in some Latin manuscripts that crept into more general usage? Or would the mistake have been more likely to happen in Greek?

Chadwick talks about “ghost-words,” or words that only exist in dictionaries and never had any real living usage, but are the result of accidents. There are several in the OED, and he claims to have found several in the LSJ. Following that train of thought, maybe we should dub this a “zombie-word”? Brought to life by mad botanists.