The same opensource app, downloadable on both stores but paid on playstore and free on fdroid. Is it legal and is it ethical? Why?
Sure , indeed there are some foss app that are the same as you described
True, but I think typically the Playstore version isn’t always from the official maintainers and I’d consider it less trustworthy, even if free (unless the devs link to the playstore page on github/gitlab/codeberg/whatever).
the play store version isn’t as trustworthy even if the original devs published it there, since google forced all developers a few years ago to hand over their signing keys. the signature is how you know the app wasn’t modified by anyone, and that actually the developer released the apk