Be careful buddy because to apply for a new, or renew, passport in Italy it's going to take up to
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months, it depends on the Questura of your area. There are a lot of delays on that matter, since I chose to apply for a new passport at the embassy of Italy in Bangkok. I had the same need to keep the old passport due to the visa stamp on it. I got the new passport at the same price (116€) but in
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weeks, just because the embassy must ask the permission to the Questura of your italian town before proceed. Afterwards I showed both passports at the Thai immigration office. They kept both the passport for one day and they moved the visa stamp from old to new passport for free.