If you’re set on testing Duet without paying, consider the (if still offered) or buying from a refundable store (Apple/Steam). Repacked IPAs simply aren’t reliable.
AltStore is a legitimate tool for sideloading open-source apps (like Delta emulator). It works via a server app on your Mac/PC. duet display ipa repack
If you can’t or don’t want to pay for Duet Display, consider these options: If you’re set on testing Duet without paying,