diff -r d3a1a092df3b src/station_cmd.cpp
--- a/src/station_cmd.cpp	Thu May 23 08:39:17 2013 +0200
+++ b/src/station_cmd.cpp	Thu May 23 10:07:32 2013 +0200
@@ -862,15 +862,16 @@
  * @param invalid_dirs Prohibited directions (set of DiagDirections).
  * @param is_drive_through True if trying to build a drive-through station.
  * @param is_truck_stop True when building a truck stop, false otherwise.
- * @param axis Axis of a drive-through road stop.
+ * @param dir Entrance direction.
  * @param station StationID to be queried and returned if available.
  * @param rts Road types to build.
  * @return The cost in case of success, or an error code if it failed.
  */
-static CommandCost CheckFlatLandRoadStop(TileArea tile_area, DoCommandFlag flags, uint invalid_dirs, bool is_drive_through, bool is_truck_stop, Axis axis, StationID *station, RoadTypes rts)
+static CommandCost CheckFlatLandRoadStop(TileArea tile_area, DoCommandFlag flags, uint invalid_dirs, bool is_drive_through, bool is_truck_stop, DiagDirection dir, StationID *station, RoadTypes rts)
 {
 	CommandCost cost(EXPENSES_CONSTRUCTION);
 	int allowed_z = -1;
+	uint num_already_built = 0;
 
 	TILE_AREA_LOOP(cur_tile, tile_area) {
 		CommandCost ret = CheckBuildableTile(cur_tile, invalid_dirs, allowed_z, !is_drive_through);
@@ -888,10 +889,17 @@
 						is_drive_through != IsDriveThroughStopTile(cur_tile)) {
 					return ClearTile_Station(cur_tile, DC_AUTO); // Get error message.
 				}
+				DiagDirection cur_dir = GetRoadStopDir(cur_tile);
 				/* Drive-through station in the wrong direction. */
-				if (is_drive_through && IsDriveThroughStopTile(cur_tile) && DiagDirToAxis(GetRoadStopDir(cur_tile)) != axis){
+				if (is_drive_through && DiagDirToAxis(cur_dir) != DiagDirToAxis(dir)) {
 					return_cmd_error(STR_ERROR_DRIVE_THROUGH_DIRECTION);
 				}
+				/* When overbuilding with the same type and direction don't count costs again for this tile. */
+				if (is_drive_through || dir == cur_dir) {
+					num_already_built++;
+					cost.AddCost(-_price[is_truck_stop ? PR_BUILD_STATION_TRUCK : PR_BUILD_STATION_BUS]);
+				}
+				/* Check if the underlying station is allowed to be joined to. */
 				StationID st = GetStationIndex(cur_tile);
 				if (*station == INVALID_STATION) {
 					*station = st;
@@ -903,7 +911,7 @@
 			bool build_over_road = is_drive_through && IsNormalRoadTile(cur_tile);
 			/* Road bits in the wrong direction. */
 			RoadBits rb = IsNormalRoadTile(cur_tile) ? GetAllRoadBits(cur_tile) : ROAD_NONE;
-			if (build_over_road && (rb & (axis == AXIS_X ? ROAD_Y : ROAD_X)) != 0) {
+			if (build_over_road && (rb & (DiagDirToAxis(dir) == AXIS_X ? ROAD_Y : ROAD_X)) != 0) {
 				/* Someone was pedantic and *NEEDED* three fracking different error messages. */
 				switch (CountBits(rb)) {
 					case 1:
@@ -956,6 +964,8 @@
 		}
 	}
 
+	if (num_already_built == tile_area.w * tile_area.h) return_cmd_error(STR_ERROR_ALREADY_BUILT);
+
 	return cost;
 }
 
@@ -1805,7 +1815,7 @@
 	/* Total road stop cost. */
 	CommandCost cost(EXPENSES_CONSTRUCTION, roadstop_area.w * roadstop_area.h * _price[type ? PR_BUILD_STATION_TRUCK : PR_BUILD_STATION_BUS]);
 	StationID est = INVALID_STATION;
-	ret = CheckFlatLandRoadStop(roadstop_area, flags, is_drive_through ? 5 << ddir : 1 << ddir, is_drive_through, type, DiagDirToAxis(ddir), &est, rts);
+	ret = CheckFlatLandRoadStop(roadstop_area, flags, is_drive_through ? 5 << ddir : 1 << ddir, is_drive_through, type, ddir, &est, rts);
 	if (ret.Failed()) return ret;
 	cost.AddCost(ret);