Is there any 5-letter English word that uses only 2 distinct letters?
This regex dictionary returns 34 matches. I removed the ones that were proper nouns or that I couldn't find definitions of:
ajaja: species name of the roseate spoonbill?
alala: ʻalalā, a Hawaiian crow.
anana: alternate name for a pineapple?
arara: a palm cockatoo.
cocco: a taro tuber.
esses: plural of "ess", the letter S.
hoooo: transliteration of Japanese 鳳凰 (phoenix).
mamma: alternative spelling of "mama", or a mammary gland.
nanna: a word for a grandmother.
pappa: a word for a father.
susus: plural of "susu", a money sharing group.
taata: a word for a father.
tatta: a bamboo frame.
uhhuh: (usually hyphenated) an affirmative response.
ululu: a wail.
unnun: to cause someone to stop being a nun.
xxiii: Roman numerals for 23 (32, 39, 41, etc. weren't listed but would also work the same).