Thanks you for your answer.
Actually, the answer was on Google's side : On the online Google calendar, in "Parameters", in the "Agendas" tab, click on "Birthdays", then you can choose what birthdays should be shown. I had both "Contacts" and "Google+" selected, I removed "Google+" and it solved the problem. What led me to think it was a Calengoo issue was that I had no duplicates in the online Google calendar, only in Calengoo - I guess the Google calendar filters the duplicates!