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Solve the following problem using Python, implementing the functions described below, one line at a time
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Description:
 def iter_user_identities(user):
 """Iterates over all existing user identities that can be used with Vidyo"""

    from indico_vc_vidyo.plugin import VidyoPlugin
    providers = authenticators_re.split(VidyoPlugin.settings.get('authenticators'))
    done = set()
    for provider in providers:
        for _, identifier in user.iter_identifiers(check_providers=True, providers={provider}):
            if identifier in done:
                continue
            done.add(identifier)
            yield identifier