def as_sender_tuple(sender):
    """build a sender tuple

    >>> as_sender_tuple('joe@testco.com')
    ('joe@testco.com', 'joe@testco.com')

    >>> as_sender_tuple(('joe@testco.com', 'joe@testco.com'))
    ('joe@testco.com', 'joe@testco.com')

    >>> as_sender_tuple(['joe@testco.com', 'joe@testco.com'])
    ('joe@testco.com', 'joe@testco.com')
    """
    if isinstance(sender, str):
        return sender, sender
    return tuple(sender)