There is no a double click event in AspMap.
To get lat/long coordinates, it is required to know what is the coordinate system of your map files. If you set the Map.CoordinateSystem property to transform the map files into a common coordinate system, then call the Map.CoordinateSystem.ToWgs84 method to transform the coordinate of the user's click to the lat/long coordinate system.
If all of your map files support the WGS84 coordinate system, then the coordinate of the user's click is already a lat/long coordinate, where
point.X - longitude
point.Y - latutude