FF_unique_user_email.php

add_action('fluentform_before_insert_submission', function ( $insertData, $data,  $formId ) {
    if($formId->id != 52) {    // You can pass your form id here
        return;
    }

    // Email based check
    $isExist = wpFluent()->table('users')
        ->where('user_email', $data['email'])
        ->first();

    if($isExist) {
        wp_send_json(['errors' => [
            'restricted' => [
                'Sorry! there are already an account registered with your email.'
            ]
        ]], 422);
    }
}, 10, 3 );