I had my passport renewed before the expiration date and now I have two valid passports [closed]

Not sure about the specifics in Italy but in most countries it works like this

1. When your new passport gets issued, the old one gets invalidated, often by punching a hole through it. But you do get it back.
2. Any valid Visas and stamps in the old passport remain valid. You just need to show both passports to immigration control. There is typically no need to transfer an existing Visa to the new passport.

CAVEAT: each country and sometimes each consulate & embassy make their own rules. You need to check and verify with the specific authorities for your case.