Through the use of the Time Turner, a person or persons can indeed be in two locations at once. It must be remembered that at no time do we see Harry or anyone else looking at the Marauder's Map while a person using the Time Turner is present at Hogwarts at the same time they are there in their normal time frame, so this is all speculative. Two schools of though prevail regarding what would happen under these circumstances:
- First school of thought is that the Map would not "see" the time-displaced person simply because it reflects the disposition of the school in "real time". (FWIW, I don't subscribe to this idea). Another version of this theory is that it would see ONLY the time-displaced person because that person is the "active" one actually making new decisions.
- The second school of thought is that both the 'current' and 'time-displaced' versions are visible on the map, since both are just as real to the Map. Now whether the Map would make any distinction in the markings (such as Hermione Granger and Hermione Granger (2)) or not (I don't believe it would), is open for debate and would depend on exactly what divinatory magic was used to allow the map to "see" people at Hogwarts. It bears mentioning that Hogwarts is a large place and has approximately 250-280 students plus staff at any given time (during school term), so actually searching and locating a second person might be difficult, even if the Map user actually knew that a second version of an individual was present.