well, i did it on purpose !i said "air conditioned" and DUO says "air conditioning" I NEVER - in all my long life (78 of which 55 traveling around this planet) used or heard " air conditioning" , always air conditioned. But i must say that my stays in the USA have always been very short, mainly stop overs going to Latin America coming from Switzerland. In French we call it AIR CONDITIONNE which is past participe , i.e. the "e" ending in English. To me " air conditioning" sounds more like the complete setting up of the system and not the individual A/C.
May I ask where you are from? Personally I have never heard "air conditioned" other than e.g. "this room is air conditioned", and even then it would often be rephrased to "this room has air conditioning". I'm from Denmark, and even the Danish language has adopted "air condition" without translation (but with the - ing part dropped).
AS I said, I am French-speaking, from Switzerland, but traveled during fifty years around the planet for business of which 26 years living in Ecuador and Peru. . Of course, in Spanish we call it AIRE ACONDICIONADO, which , grammatically, is PAST PARTICIPE, like in French AIR CONDITIONNE . In the hotels I always asked if they had air conditioned, never air conditioning. Nobody ever said anything. It is,of course ,easier to say A/C.
I never heard air conditioned like that either. Grammatically that makes no sense when you say "This room has air conditioned" instead of "This room has air conditioning".
In American English, the standard terminology is indeed air conditioning.