Fixed: in the Magpie app, signing in with Apple, Google, LinkedIn or GitHub to an account whose email address had never been confirmed signed in at once and linked the sign-in method to the account. If someone else had registered that address with a password, both of you could then use the account.
The app now shows Check your sign-in methods first, as the website does. It lists what is set up on the account and its recent sign-ins. Keep them and continue keeps everything; Remove them removes everything listed and signs out every other session on the website, in the app and in the browser extension. Either choice confirms the address, links the sign-in method and signs you in. An account with nothing set up is confirmed without the check.