Freeform user account creation form: handling prior user account exists

Question

This is more or less a duplicate of a support ticket raised with Solspace. I’ve no doubt they will provide an answer but if anyone else has one feel free to pitch in.

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I'm using Freeform forms to register user accounts into a specific user group per form. It could arise that a would-be user is already registered, so I need to handle them trying to create a user account for an email address already in the system.

It appears that form.hasErrors prevents a duplicate account being created but doesn’t return any message that that’s what has happened within the form.errors loop.

I'm imagining I could do something like the following to assume that a prior user account is the issue, but it would be good if there was something more systematically robust:

{% if form.hasErrors %}
    {% if form.errors|length %}
        {# Report errors #}
    {% else %}
        {# Assume that an existing user account is the issue #}
    {% endif %}
{% endif %}

Is there a better way?