Sound goes down when another in app makes a sound

[Juerg2]

I like to listen to music on spotify while I learn languages with Duolingo. But it us pretty annoying that the volume keeps changeing for a split second when ever Duolingo makes a sound. I put the volume low enough to understand the other app, i dont want it to automatically lower. Does anyone know what might cause this and how to get rid of it? Thanks

I use android 8