FormValidation.php

<?php

class Page_Controller extends ContentController {

    private static $allowed_actions = array(
        'MyForm'
    );

    public function MyForm() {
        $fields = new FieldList(
            TextField::create('Name'),
            EmailField::create('Email')
        );

        $actions = new FieldList(
            FormAction::create('doSubmitForm', 'Submit')
        );

        $form = new Form($controller, 'MyForm', $fields, $actions);

        return $form
    }

    public function doSubmitForm($data, $form) {
        // At this point, RequiredFields->validate() will have been called already,
        // so we can assume that the values exist. Say we want to make sure that email hasn't already been used.

        $check = Member::get()->filter('Email', $data['Email'])->first();

        if($check) {
            $form->addErrorMessage('Email', 'This email already exists', 'bad');

            return $this->redirectBack();
        }

        $form->sessionMessage("You have been added to our mailing list", 'good');

        return $this->redirectBack();
    }
}