As for my device, whenever I play duolingo and music at same time then the audio fades slightly with CM 12 builtin player

Now if you are Ok with any third party application. Here is it what you need to do. 

1. Install [Poweramp Music Player ][1].
2. Go to its settings.
3. Then click on audio.
4. Select Audio Focus.
5. Uncheck Short Audio Focus Change.
 
[![Here is the screenshot][2]][2]


  [1]: https://play.google.com/store/apps/details?id=com.maxmpz.audioplayer&hl=en
  [2]: https://i.sstatic.net/XXCmW.png