The deviceAdded signal was emitted twice, once with the old device list
in m_deviceList, once with the new one.
The first one is actually wrong and was fixed in kdelibs4 by moving the
deviceAdded signal after the m_deviceList update. In KF5, the second
signal was added but the first one never removed.
Move the m_deviceList update to the top (though for deviceRemoved it
does not matter much), and remove the second signal.
Depends on D28779