Steps to reproduce

1. Try to register twice with the same mail adress with the public user registration module. 

Expected results

I should get a notification that the email adress already exists.

Actual results

An error occurs when I register twice with the same email address. When I type in an email address that already exists and I click on the register button, a new page is loaded that displays an error message that the registered email address already exists (as it should be). But an user might very likely then click on "back" in the browser. When he does that the data entered in the registration form is still there. If he then changes the email address and clicks on register again, an "HTTP Status 403 - Forbidden" message appears (if he would refresh the page it would work, but how would he know?)

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Workaround

There is no workaround known.

Development notes