This problem is already solved, so I'm just posting this as an FYI for anyone else who runs into it.
I have a form setup on a custom login page, and it worked fine for awhile. But, after upgrading to one of the recent versions of Formidable, I started getting the generic form failure error -- "We're sorry. It looks like you've already submitted that." -- whenever the form was submitted. Forms submitted from normal WordPress pages continued to work fine.
All the other support threads I read said that it was probably a corrupt database, but emptying my database tables didn't fix the problem.
I eventually realized that
FrmEntriesController::process_entry() wasn't getting called (probably because the
wp action doesn't get triggered on wp-login.php), so I manually triggered the
frm_wp action before calling the FrmEntriesController::show_form() and that fixed it.
do_action('frm_wp'); if( method_exists( 'FrmEntriesController', 'show_form' ) ) echo FrmEntriesController::show_form( 2, '', true );