The system will only charge from one port at a time (in fact it's not clear the notebook will charge over USB-C at all, since the system needs either a 180W or 240W AC adapter to itself).    There will be no ill effect from keeping both the OEM adapter connected to the notebook and a separate one to the docking station (in fact that's the only way both will run, I suspect you'll find).  
Hello, i have recently acquired a WD19tb dock second hand and wish to use it with my percussion 7670. I aware it cant charge my laptop efficiently.  The dock came with a 240w PSU (I'm guess isn't original but is a dell PSU much like the 7670 psu) which is need to make it work. is it ok to have the laptop 240psu plugged in as well or is there another way, like plugging a second usb c in or something from the dock? from the thunder bolt port. I don't often max out the pc as it mainly used in a CAD support role.    so what I'm asking is should I just plug both in and is there no ill effects or is there an alternative solution with my set up. I'm aware there are bigger and badder docks out there now but this was available on a well know market place.  I wasn't aware the dock need power to work and was just going to use as an adaptor hence the question above
dell precision 7670 and dock WD19TB