apparently polish people get so little verification that a friend of mine signed the petition 14 times with his name, adding a random letter each time

Probably specific by country then because I didn't have to show any ID. FWIW

"To sign, you must provide a set of personal data, which is required by the authorities of your country for verification purposes." which is as of a week ago and still today :

• nationality
• name
• birth date
• address

that's good to know. when I signed last year I think it was enough to type in my id card's number and my name and location.

Not necessarily. But nontheless fraud will be detected when the local governments check on that.