Have you ever seen a guy with this much XP? He's hacking for sure - look at the second image, where all of his courses are shown. I calculated how much XP he must have gotten every day for this (assuming he didn't skip a day since the beginning of his streak) and how long it would've taken him, were he legit.
I found that he would've had to get at least 4000 XP a day, every day, to have such a large total. For comparison, my highest amount of XP in one day was ≈5300 (It took me 3 hours).
Now, I wanted to find out how long it would take me to get these 4000 XP, assuming that there isn't too much variation in the grinding method used. I found a very good method for getting XP fast (not that I always use it, but yeah... it works) and it takes me ≈12 mins to get 1000 XP using it. This means that this guy actually might be legit.... right?
Well, the first thing that points in the direction of cheating is that this 1000 XP in 12 mins is earnt using my 2x XP boost from the golden chest daily quest. Otherwise I would have to complete one of the stepping stones on the path of my course to get a double XP boost (I use PC version, so I won't be factoring in 3x XP boost because I can't get this on computer but also because the easiest XP grinding method I found only works well on computer). Seeing as a session for this guy would average around 3/4 of an hour of the 2x XP boosts (15 mins each) he would need around 15 extra minutes or so to get the other two XP boosts as you can only get one of the three needed from doing quests.
Another thing, if you look at the first image with his profile on it, he hasn't been getting 4000 XP recently. His average XP per day in the last week was 12,000?!?!?! This just seems too unrealistic, seeing as a friend of mine, who is completely legit by the way, once got 10,000 XP in one day and it took AGES. How do you AVERAGE these types of scores each and every day without cheating? It would take close to 4 hours (if you are getting XP quite quickly) to get these types of numbers every day. I just don't see anybody doing this every day, for multiple years, all by himself.
I don't know how this would work, but I'm pretty sure that if multiple people logged into one Duolingo account, and grinded simultaneously, this could work out.
But still, that would take around 12 reasonably grindy people to get this much XP (if they got 1000 each) every day.
Anyway, this is interesting to see, and I hope that I can uncover tHe WaYS oF ThE cHeATer (and stop them from winning the leaderboards each week as it is super annoying)!