Return error when upgrading airport to an identical configuration

See: #200

src/station_cmd.cpp

@@ -2467,6 +2467,10 @@ CommandCost CmdBuildAirport(TileIndex tile, DoCommandFlag flags, uint32 p1, uint
 		return_cmd_error(STR_ERROR_TOO_CLOSE_TO_ANOTHER_AIRPORT);
 	}
 
+	if (action == AIRPORT_UPGRADE && airport_type == st->airport.type && layout == st->airport.layout && st->airport.tile == tile) {
+		return_cmd_error(STR_ERROR_ALREADY_BUILT);
+	}
+
 	/* The noise level is the noise from the airport and reduce it to account for the distance to the town center. */
 	uint dist;
 	Town *nearest = AirportGetNearestTown(as, iter, dist);