Use a sideloading utility (AltStore/Sideloadly) to sign the IPA with your Apple ID.
However, Duet Display is not free. While the basic app requires a one-time purchase (or a subscription for the "Pro" features), many users search for a loophole: the duet display ipa repack
Often patched to unlock premium features like "Duet Pro" (Apple Pencil support, wireless connectivity). Use a sideloading utility (AltStore/Sideloadly) to sign the