Change no-duplicate validation message on email field

Hello,

I'm trying to change the no-duplicates validation message on my email field.

But no luck, the custom validation message field doesn't seem to have and options or doing much at all.

The only thing I could find on change validation messages by a script was this...

<script type="text/javascript">
     jQuery(document).ready(
          function() {
               jQuery("#gform_21 .validation_error").html("My new validation message");
     }
);
</script>

But I can't see this working with my AJAX form, and there no options to say whether it's a validation message for a duplicate or an empty field.

For example:
If the Email field is not filled in, I need it to say "Required Field" ...
...and if the email field is a duplicate, then I need it to say "Already Registered"

as opposed to the current default...
"This field requires an unique entry and 'example@mail.com' has already been used"

Any help soon would be great thanks.