Index: src/newgrf_commons.cpp
===================================================================
--- src/newgrf_commons.cpp	(revision 12066)
+++ src/newgrf_commons.cpp	(working copy)
@@ -107,30 +107,32 @@
  */
 uint16 OverrideManagerBase::AddEntityID(byte grf_local_id, uint32 grfid, byte substitute_id)
 {
-	uint16 id = this->GetID(grf_local_id, grfid);
-	EntityIDMapping *map;
+	uint16 first_free_id = invalid_ID;
 
-	/* Look to see if this entity has already been added. This is done
-	 * separately from the loop below in case a GRF has been deleted, and there
-	 * are any gaps in the array.
+	/* Search all IDs for a free slot, and test if it has already been added.
+	 * Note: In case a GRF has been deleted, there can be gaps in the array
 	 */
-	if (id != invalid_ID) {
-		return id;
-	}
+	for (uint16 id = max_offset; id < max_new_entities; id++) {
+		EntityIDMapping *map = &mapping_ID[id];
 
-	/* This entity hasn't been defined before, so give it an ID now. */
-	for (id = max_offset; id < max_new_entities; id++) {
-		map = &mapping_ID[id];
+		/* Has the entity already been added? */
+		if (map->entity_id == grf_local_id && map->grfid == grfid) return id;
 
-		if (CheckValidNewID(id) && map->entity_id == 0 && map->grfid == 0) {
-			map->entity_id     = grf_local_id;
-			map->grfid         = grfid;
-			map->substitute_id = substitute_id;
-			return id;
+		/* Find and save first free slot */
+		if (first_free_id == invalid_ID && CheckValidNewID(id) && map->entity_id == 0 && map->grfid == 0) {
+			first_free_id = id;
 		}
 	}
 
-	return invalid_ID;
+	/* The entity has not yet been added, add it to the first free slot (if there is one) */
+	if (first_free_id != invalid_ID) {
+		EntityIDMapping *map = &mapping_ID[first_free_id];
+		map->entity_id     = grf_local_id;
+		map->grfid         = grfid;
+		map->substitute_id = substitute_id;
+	}
+
+	return first_free_id;
 }
 
 /** Gives the substitute of the entity, as specified by the grf file