`DuplicateUniqueItem` when accessing a webshare with username that is not unique across every internal LoginMethod

[jonathanj]

Reading the documentation it seems like this ought to be impossible:

def linkTo(sharedProxyOrItem):
    """
    Generate the path part of a URL to link to a share item or its proxy.

    @param sharedProxy: a L{sharing.SharedProxy} or L{sharing.Share}

    @return: a URL object, which when converted to a string will look
        something like '/users/user@host/shareID'.

But in reality linkTo generates a URL with only a localpart username, and _storeFromUsername only expects a username with a localpart.