The pronunciation of "coffi" is way off.
On www.ivona.com it is pronounced correctly by 'Welsh, Geraint', but 'Welsh, Gwyneth' , as used by Duo, seems to overlay some other word at the same time - this happens with a handful of other expressions, too.
I've had a play around and it seems like only happens with "dych chi'n hoffi coffi?". "Dych chi'n eisiau coffi?" and even "wyt ti'n hoffi coffi?" don't have the glitch. Weird.