From what I understand, as a Catholic to be baptised, you have to be named after a Christian saint, so if your first name is not a name of a Christian saint, your parents have to pick you a 2nd given name that comes from a Christian saint. But in Poland we call it just a 2nd name, since we have different words for a first name and for a last name, so there's no confusion.  

 And a lot of people get a 2nd name even if their 1st name comes from a Christian saint already.
I'm one of the minority of people without a 2nd name here and it's saving me from some headaches with the documents.