I think, when you form Rome, the decision to form Italy should vanish.
As an exploit, I could go between Italy and rome, getting the base tax bonuses. I did it over a 100 times and had thousands coming in every year.
Also, if you leave the Roman Empire and load up as someone else, the AI will form back into Italy and stay that way.
Same things happen with Celtica-Great Britain
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im not exactly known with the triggers of Rome, but make sure this same doesnt happen with Two sicilies.