Saw someone go from 8,000+ points to over 11,000 points in minutes. Kinda makes the competitiveness worthless...
Take a look at their https://www.duome.eu/ page (so add their username at the end of that link). Click on "raw" above their progress. If it moves in huge steps and/or the person finishes more than one lesson in the same second, chances are that they are cheating. (An example for that can be seen in this post: https://forum.duolingo.com/comment/35332798 )
If someone really seems to be cheating, you can report them under this link (taking a screenshot of their progress on duome can help): https://support.duolingo.com/hc/en-us/requests/new
And yes, people cheating or gaming the system really take the fun out of it. I have someone in my league who got over 8000 XP last Sunday, so I already know it's pretty pointless to try to reach the top of the league just by learning. :/