"Non l'ho mai detta."
Translation:I have never said it.
Because of the form "detta" it can only be Duolingo's translation. It is a rather difficult concept: non l'ho detto = I (have) not said it. "I did not tell him" is to be translated as "I did not tell him it." "Non gliel' ho detto." Where "glielo" means "(to) him it". Incidently: for "her" you should use the same form in this phrase. (sorry) You know of course that participium perfectum in specific situations shows the gender of the word it refers to. "Si è sedutA"= SHE sat down. "Si è seduto"= HE sat down. For the use of dettA see my other post.
It is a bit tricky, indeed .. if the present perfect is built with "essere" the participle has the gender of the subject. If it is built with "avere" and direct object, there is no change. If the object is replaced by a pronoun, then the participle carries the gender of the pronoun.
I was born in Ft. Oglethorpe, GA and lived near there until I was almost 19. There's definitely a distinctive set of dialects there, and strong class implications associated with each one.
One of my cousins yelled at his kids for playing too loudly, saw me grinning at him, smiled back and said, "It's not like none of us never done nothing like that." It may be the only quadruple negative I ever found in the field, and all it meant was "I know; we did that too." But I don't expect Duolingo to accept dialect.
That said, I do think I can imagine a standard speaker saying "I have not ever said it!" with a strong emphasis on "ever." (Per your suggestion.) Still, I think that's far enough from mainstream use that I wouldn't expect Duolingo to accept it. Still, it wouldn't hurt if they did. They accept worse things.