I can see this going one of three ways. One is that you replace your tile with the one already there. You might place the old tile in your pile to use again later. This keeps the game moving indefinitely. The second option is to stack your tile on top of the existing one. This has the effect of limiting the map size, possibly to a very small size. The third option has one--or a limited number of--options for how the original tile can be altered. The tile could be flipped to the second option, or an additional marker--such as ones for flooding or fire--is placed on top of it.