Quantum i believe reading somewhere here in the past that f-droid hosts a version on their website (say x.x.a), that get's auto updated after installation (to x.x.c).
Op now has x.x.c. installed. On his other profile he's downloading the app from the same website, but he is initially trying to install x.x.a, which is lower and is blocked because of downgrade protection, hence the error.
Other than installing in owner and pushing to respective profiles, i myself don't really know a solution for apps that behave this way.