in VS2019 and also in VS2017 you can select Tools->Options->Keyboard and you can choose in the scheme drop-down Resharper (Visual Studio) option, and you will get a mapping like if you were using Resharper, in this case Ctrl + D will do the trick for duplicating the line. Anyways, based on the documentation this should be out of the gate in Visual Studio 2017 Version 15.8 or greater