Formerly - Checking if e-mail already entered
(this is more of a jquery question than Craft, but...:))

The reason this doesn't work is because $.post works asynchronously (that's the A in AJAX). It creates a request, with a success handler (the function), then the submit handler continues execution. The success handler isn't run until the request completes, which is probably a couple of hundreds of milliseconds later.

There are various ways to solve this, but the basic idea would be to cancel the submit event straight away if the check hasn't been done, do the check, and then submit the form again if the email didn't exist, and not run the check the second time.

Here's something a bit ugly that I think'll work, but it can be done much more elegantly, I'm sure:

var emailWasChecked = false;

$("#form").on('submit', function (e) {
    if (!emailWasChecked) {
        e.preventDefault();

        $.post('?ok=1', {
                action: 'formerly/submissions/alreadySubmitted',
                formid: $("input[name=formId]").val(),
                email: $("#formtest_email").val()
            },
            function (data, status) {
                if (data.alreadysubmitted != true) {
                    emailWasChecked = true;
                    $("#form").trigger('submit');
                }
            });
    } else {
        emailWasChecked = false;
    }
});