Say each name with an Italian accent.
They sound the same! In a language where "capacola" is pronounced "gah-bah-GHOUL" there are many ways to say the same thing without realizing it and even more ways to spell it. Remember too that, Italy, as we know it, was not one unified country until relatively recently. In antiquity it was divided into several regions which had their own distinct language and traditions. There are areas where one local dialect is so foreign to another that the countrymen cannot understand each other.
Diana, it has been claimed, is just an imported version of Artemis. Janus, it is said, was a Deity native to pre-Roman Italy and worshiped above others before the Romans took over. Both Dianus and Jana are allegedly their opposite aspect, not their consort but more like an Intelligence.
Do you have an opinion? Insight? Personal Gnosis?
I'm not ready to post my research and conclusions. I am once again reading The Aeneid. I've just started. There are free (legal) copies on line if you'd like to join me!