Yeah they did not stick with this one. They are not even satisfied with just this one. They have actually made a "duolingo plus", and on iOS even a "hearts system" (not limited to tests, but overall accessibility of any lessons). So basically they have combined all the business models into one single app (ads, freemium, lives system, paid currency). It's quite disappointing how Duolingo ended up being.
I believe that it is an artifact of the speaker trilling the R at the beginning of a word. I'm used to hearing it from some Persians, and a few folks from Southwest France/Andorra, and it carries over into other languages too.
If the apparent la doesn't make sense, then ignore it. If it's apparent existence still confuses you after a few cycles through the lesson, then report it and complain.
Mortu reklamojn, mortu!
(Piedbatu la sitelon estas sensensaĵo en Esperanto)
And MY take on the forsoothly here would be Thou shouldst kick yon bucket, ye twice poxed advertisements. 'Twas always the second person which receiveth the ~st ending.
(Yes, I did some time on the Renaissance Faire circuit.)
As I said, I spent a few years being an actor at Renaissance Faires. The big mistake most amateurs make there is thinking that the archaic endings go on verbs which relate to the wrong person. I kick thy arse, Thou kickst her, She kicketh me, etc. Dig out your King James Bible and study thou the speech patterns therein, after a time it cometh naturally.