If a personal name ends in a consonant (except -er) then a termination
of
two "-i-"s must be added for the proper genitive.

Iris Cohen schreef
I got that one, but please explain Cedrus libani. It is not a person's
name, but why didn't they call it libanensis or libaniana?
Iris,

+ + +
As you note "libani" is a genitive also. It is not a genitive based on a
personal name and thus not augmented with the honorific extra "-i-". Some
personal names are not either, since traditionally this only happened to
distinguish from 'surnames' from given names. There are still some
traditional given names which also are spared the extra "-i-" (eg
"francisci")

This "libani" is a genitive based on a geographical name. Traditionally both
genitives and adjectives were used for geographical features, but at some
point there slipped in the recommendation that it should be an adjective
(now 60D). I am not sure why, likely to avoid confusion: it appears as if
"francisci" always is from a person and "franciscana" from a town/city St
Francis[co].
PvR