Consider the everyday words accelerate, accept, accident, success, vaccine. They demonstrate the English spelling-to-sound principle that where double -cc- is followed by e or i the pronunciation is ks. Thus we have əkˈseləreɪt, əkˈsept, ˈæksɪdənt, səkˈses, ˈvæksiːn.
There are two rather rarer words where not all speakers follow this principle. One is flaccid, where ˈflæsɪd competes with the expected ˈflæksɪd. The other is succinct, where I have just heard an American narrator on the BBC pronounce səˈsɪŋkt rather than the səkˈsɪŋkt that I would say myself. I can think of no reason why these two words should be exceptions to the general rule. Like the others, they are of Latin origin. Their pronunciation goes back to a Latin double -cc-, which was classically kk but subsequently had the second velar ‘softened’ when it developed into tʃ or s in late Latin and the successor Romance languages.
As you would expect, this being English spelling, there are a handful of other words that violate the rule. Soccer ˈsɒkə, ˈsɑkɚ obviously “ought” to be spelt socker. And lovers of classical music will be familiar with Italian names such as Puccini puˈtʃiːni and Pagliacci.
Strangely, the corresponding rule doesn’t work in the case of the voiced etymological equivalent. In exaggerate ɪɡˈzædʒəreɪt we have simple dʒ, not *ɡdʒ. In suggest Brits have simple dʒ, thus səˈdʒest, but most Americans have ɡdʒ, thus səɡˈdʒest. Otherwise the velar remains unsoftened; there don’t seem to be any other Latin-derived words with -gge- or -ggi-. Double gg stands for simple g in occasional non-Latin-derived words such as druggist and digging. And then there’s ciggy ˈsɪɡi, colloquial abbreviation of cigarette, which demonstrates the use of -gg- in informal spelling.
If you’d like to wallow further in the irregularities and inconsistencies of English spelling, allow me to recommend Masha Bell’s blog.