“Afghani” is an apparent conflation of “Afghan” with “Pakistani” and reflects the general tendency to confuse Afghanistan with Pakistan. It’s conceivable that people who say “Afghani” are trying to steer clear of the term “Afghan” because it makes them think of a wool coverlet or a large dog, associations that they fear may give offense. Nonetheless, “Afghan” is the correct term.
“Afghani” is a real word, but it refers not to the people of Afghanistan, but rather to its principal unit of currency. Hence, referring to the people of Afghanistan as “Afghanis” is roughly equivalent to referring to the people of the United States as “dollars.”
FICTION!