It is a glitch in the computer-generated voice used on Duo. Generally it is pretty good, but there are occasional oddities such as this one - this is pointed out in the first set of 'Hints and Tips' in the course and elsewhere.
The voice comes from www.ivona.com. If you go to that web site you can put any Welsh text into the box on their home page and then choose either 'Welsh, Geraint' or 'Welsh, Gwyneth' to speak it. 'Welsh, Geraint' will sometimes give a better rendering than 'Gwyneth'.
If you go to www.gweiadur.com, a very good on-line dictionary, you will often find a speaker symbol next to the Welsh head-word - if you click on that symbol you will hear a natural voice recording of the pronunciation. (Gweiadur requires you to register for free - a bargain!)
I have always in primary school wondered deeply why 'tost' means 'toast', but also 'sore'. It still confuses me. Mind you, maybe it's referring to the fact that perhaps, when the bread is inserted into the toaster, this causes great pain as the innocent wholemeal slice brushes past the hot metal skewerlines.
Shouldn't this be spelled with a circumflex? All other resources I've seen outside of Duo spell it as such. It even tells me it's a spelling "mistake" if I include it...
Given the circumflex indicates the long vowel with which this word should be pronounced, I don't really understand.