The best alternatives to Duet are Ocean beta, Super Looper, and Pulse. If these 3 options don't work for you, we've listed a few more alternatives below.
Ocean lifts the barriers that are often associated with electronic music-making & enable people to enjoy making music, exploring sounds and collaborating with people globally.
The app enables you to work with musicians around the world on multi-track music videos. One artist can start a song and then any other artist can edit it to add their own tracks. It's sort of a mashup between Smule and GitHub. Try it and see what you create.