qlocalsocket_win.cpp:129

if (state() == ConnectedState || state() == ConnectingState)
return;

If a user wait for some reaction on connectToServer() call, he will never get it. He must check the same condition himself, to make some workaround. Thus this condition will be checked twice. Every time.
Either an error signal should emit, or some error value should return to avoid this.
Btw, QAbstractSocket::connectToHost() function has the same feature.