The x-axis being log scale by default visually exadurates the effect if you are thinking about it in terms of area under the curve. This confused me also.
The reason there are two Gs is that the first one is assimilated from a D. The etymological structure of the word is ex-ad-gero,¹ "from-toward-carry".
This happens for everything. Compare accelerate [ad-celero] / accommodate [ad-con-modus] / impossible [in-pot-s-ibilis] / intelligent [inter-leg-ents].
(Note that a doubled consonant in Latin is pronounced differently from a single one, as is still true today in Italian. English draws no distinction, which makes the etymological spellings likely to confuse English speakers.)
¹ This was already the form of the word in Latin. It looks malformed to me; I can't explain why exaggero is a first conjugation verb (with passive participle exaggerata) rather than simply being a prefixed form of its root (ag)gero, in which case the participle would have been exaggesta. Gero and aggero are both third conjugation verbs...
I think exaggero is a denominative, derived from the noun agger 'pile' (itself a nominalisation of aggero i.e. "what is brought"). As such it took the default (productive) first conjugation. You can see a similar effect in English denominative <to highlight> which takes the productive past tense suffix -ed in highlighted, not *highlit.
