Merge branch 'master' into jgrpp

# Conflicts:
#	regression/regression/result.txt
#	src/aircraft_cmd.cpp
#	src/airport_gui.cpp
#	src/articulated_vehicles.cpp
#	src/console_cmds.cpp
#	src/date_gui.cpp
#	src/engine.cpp
#	src/genworld_gui.cpp
#	src/gfx_layout_fallback.cpp
#	src/group_gui.cpp
#	src/hotkeys.cpp
#	src/network/core/tcp_connect.cpp
#	src/network/core/tcp_listen.h
#	src/newgrf.cpp
#	src/newgrf.h
#	src/newgrf_engine.cpp
#	src/newgrf_gui.cpp
#	src/newgrf_station.cpp
#	src/openttd.cpp
#	src/order_gui.cpp
#	src/os/macosx/osx_main.cpp
#	src/pathfinder/yapf/yapf_node_rail.hpp
#	src/rail_gui.cpp
#	src/saveload/afterload.cpp
#	src/saveload/cargopacket_sl.cpp
#	src/saveload/linkgraph_sl.cpp
#	src/saveload/station_sl.cpp
#	src/script/api/script_industrytype.cpp
#	src/settings.cpp
#	src/settings_gui.cpp
#	src/settings_table.cpp
#	src/settingsgen/settingsgen.cpp
#	src/station.cpp
#	src/station_cmd.cpp
#	src/strings.cpp
#	src/timer/timer_game_calendar.cpp
#	src/timer/timer_game_calendar.h
#	src/timer/timer_manager.h
#	src/timer/timer_window.cpp
#	src/timetable_cmd.cpp
#	src/toolbar_gui.cpp
#	src/town_cmd.cpp
#	src/town_gui.cpp
#	src/train_gui.cpp
#	src/vehicle_cmd.h
#	src/vehicle_gui.cpp
#	src/viewport.cpp
#	src/widgets/dropdown.cpp
#	src/window_func.h
#	src/window_gui.h

src/fileio.cpp

@@ -14,6 +14,7 @@
 #include "fios.h"
 #include "string_func.h"
 #include "tar_type.h"
+#include "3rdparty/cpp-btree/btree_set.h"
 #ifdef _WIN32
 #include <windows.h>
 # define access _taccess
@@ -84,6 +85,8 @@ static bool IsValidSearchPath(Searchpath sp)
 static void FillValidSearchPaths(bool only_local_path)
 {
 	_valid_searchpaths.clear();
+
+	btree::btree_set<std::string_view> seen{};
 	for (Searchpath sp = SP_FIRST_DIR; sp < NUM_SEARCHPATHS; sp++) {
 		if (only_local_path) {
 			switch (sp) {
@@ -97,7 +100,11 @@ static void FillValidSearchPaths(bool only_local_path)
 			}
 		}
 
-		if (IsValidSearchPath(sp)) _valid_searchpaths.emplace_back(sp);
+		if (IsValidSearchPath(sp)) {
+			if (seen.count(_searchpaths[sp]) != 0) continue;
+			seen.insert(_searchpaths[sp]);
+			_valid_searchpaths.emplace_back(sp);
+		}
 	}
 }