My situation:

Amazon unlimited playlist containing 12 songs. 1 of those songs is repeated. 
when trying to play either ‘version' of that song within the playlist it fails saying error with encoding. 
if I create a new playlist with all songs just once they play perfectly. 
this coincides with me accepting the HD upgrade from Amazon music. 
 

the simple workaround is to only ever have a song in your Amazon music playlist once but it used to work, for years on the same playlist. It also means if you accidentally add your favourite song twice you’ll never hear it. 
 

Anyone else having this issue? Know if there is a plan to fix it?