Please go to Personal - Settings - Linked accounts to link your Facebook, Twitter, Google or Line accounts together to one Castbox account. Then you can use any of them to log in.

Please notice that since Castbox offers different login methods, if you selected different entries to sign in instead of linking them inside the app, Castbox will recognize them as different accounts. If you have accidentally created another Castbox account using your Twitter, you will not be able to link it inside the app to the other Castbox account.