Can I plug my 15" 2017 MacBook Pro into a USB-C display while using the charger cable? I've bought myself a Dell U2719DC display, which allows your device to charge when connected via USB C. However the maximum charging ability is capped at 65W, whereas the stock 15" MacBook Pro charger is 83W, so while plugged in, the computer will still charge, just slower.

Was just wondering if having them both plugged in and supplying power, is going to do any harm? Are the MacBook Pros smart enough to manage their power input?
It shouldn't be a problem.  I did the same thing with an LG 5K display, and the power plugged in separately, and had no issues.

Having written that, I almost never bothered to do it that way, because my laptop spent most of it's time on my desk, and I never needed to charge it faster.