The system will only charge from one input at a time -- if you plug in the adapter it will override the USB charging from the docking station.  
I have Dell Precision 5540, which is charging from its original charger. And recently I have plugged that with a Docking Station to share my screen to two different screens. Now seems like both my Laptop Charger and Docking Station are charging my laptop. My Question is, is this dangerous will not overheat the battery ??.. Also If I remove my Original laptop charger and restart PC, plugged with docking station at BIOS Level when PC is restarting it give message that docking station is not supplying enough Power, laptop will still run but can be slow.
Effects of Dual Charging to Laptop with Original Charger and Docking Station