Mariah Carey wasted no time proving what we told you 2 weeks ago -- there's only room for one diva on "American Idol."

Carey and Nicki Minaj went at it during the first "A.I." taping Sunday in NYC. When Nicki started critiquing a contestant, Mariah would interrupt -- not once, but many times. Each time Mariah interrupted, Nicki fought back by loudly talking over Mariah.

One "A.I." spy said Mariah and Nicki tried to cover their disdain for each other but everyone saw through it, adding, "These girls just don't like each other."

You may recall, we reported when producers called Mariah to tell her that Nicki would probably be joining her as a judge, she hung up the phone.