"Friends and family."
Translation:Caraidean agus teaghlach.
They are basically old and new versions of the plural of the same word.
Dwelly (1911) gives càirdean without comment
Am Faclair Beag (current) gives caraidean without comment
But Mark (2003) says
caraidean is now often used in place of càirdean except in the vocative, although this appears to be quite unnecessary
I had not noticed this but I think I would always use a chàirdean in the vocative. So basically both are correct and I am sure that eventually, if not now, Duo will accept both. Note however, that if you get it wrong or make a typo, Duo does tend to be a bit random as to which it says you should have typed.