Two n's in laciniate?
stress-doubleA doubled consonant would mark a short stressed vowel. Laciniate has its stress on an open syllable and doubles nothing.
heard aslacinniate
spelledlaciniate
saidluh-SIN-ee-ut
One. laciniate.
Doubling the n would put a short stressed vowel in front of it, as in cannon or beginning. The stressed vowel here comes before the n, not after it, and the doubling instinct has simply fired one letter too late.
More of the same word
lacciniate laciniat luciniate leciniate lasiniate lacineate
Every one of them has an entry. All 9 spellings of laciniate.