I accidentally typed "k" and it still accepted the answer. Why?
my guess: for English numbers it accepts digits, i.e. "7", so due to Dou's general one-letter-wrong-spellchecker, any single alphanumeric character is accepted, with just a misspelled-warning.
You're right. Duo has a big problem with handling this. I checked in the incubator and 'k' would still be an accepted answer, as well as 'l' or '+', but for instance 'p' wouldn't work. It's pretty weird :)