-User clicks to register on mobile app with twitter -User enters duplicate email Should the user be able to update email on the same validation screen? or should he be informed only about the duplicate email with being directed to splash screen to start registration process over again
Send a mail to the user.
If the email address is used, inform the user of an attempt to login, and link him to his already existing profile.
If the email address ain't used, link the user to a confirmation screen and initiate a welkom flow on your app.
Never show if an email address is already used, my wife could for example find out that I am subscribed on datingsites without needing any access to my email account. That might be valuable information for not only my wife.