It fills in the gaps, but does not give you a "copy" of the content that you already have. Basically, if I already owned Munich Garmisch, and I bought the TS15 Steam Deluxe Edition, I'd get the rest of the content from the package itself. Steam is unified, meaning that you do not receive extra copies of anything. Everything still falls under "Train Simulator 2015."
Same thing goes if you were to invest in the Packages (which I highly recommend you take a look at, as they save you money. Welcome to the world of a money-hungry hobby lol.