Whenever I move my map in my application I download new annotations(or coordinates rather) from a webserver, appropriate for the maps state.
I do not want to download already downloaded annotations again. I need a smart, selective way to download the new ones, avoiding duplicates.
You could grid off the map into sections and make requests based on tiles. that way you know if a tile has been downloaded or not and you can only request the tiles that are not yet populated.