「きたないです」 does that not mean "It is not dirty" because of the ない?
'きたない' is 'dirty'.
'きたなくない' is 'not dirty'
Don't worry, Duo is correct. ( ´ ▽ ` ) But I understand your feeling. Certainly.
If you look at the Kanji, きたない = きたな+い = 汚+い=汚い, not きた+ない
送り仮名/おくりがな/okurigana is ambiguous a bit. (･･;) both are used.
きたない is simply a non-negative adjective meaning "dirty". It's not related to the ない used for negation.
Why not "これは汚い"?
That means "this is dirty"