def test_add_already_added(self):
        prio_set_list = event._PrioritizedSetList()
        obj = object()
        prio_set_list.add(0, obj)

        with pytest.raises(ValueError) as excinfo:
            prio_set_list.add(0, obj)
        excinfo.match(r"has already been added")

        with pytest.raises(ValueError) as excinfo:
            prio_set_list.add(1, obj)
        excinfo.match(r"has already been added")
async def echo_server(loop, address, unix):
    if unix:
        sock = socket.socket(socket.AF_UNIX, socket.SOCK_STREAM)
    else:
        sock = socket.socket(socket.AF_INET, socket.SOCK_STREAM)
        sock.setsockopt(socket.SOL_SOCKET, socket.SO_REUSEADDR, 1)
    sock.bind(address)
    sock.listen(5)
    sock.setblocking(False)
    if PRINT:
        print('Server listening at', address)
    with sock:
        while True:
            client, addr = await loop.sock_accept(sock)
            if PRINT:
                print('Connection from', addr)
            loop.create_task(echo_client(loop, client))