Fixes TileSet set as local to scene

doc/classes/TileSet.xml

@@ -68,6 +68,7 @@
 			<description>
 				Adds a [TileSetSource] to the TileSet. If [param atlas_source_id_override] is not -1, also set its source ID. Otherwise, a unique identifier is automatically generated.
 				The function returns the added source ID or -1 if the source could not be added.
+				[b]Warning:[/b] A source cannot belong to two TileSets at the same time. If the added source was attached to another [TileSet], it will be removed from that one.
 			</description>
 		</method>
 		<method name="add_terrain">

doc/classes/TileSetSource.xml

@@ -8,6 +8,7 @@
 		Tiles in a source are indexed with two IDs, coordinates ID (of type Vector2i) and an alternative ID (of type int), named according to their use in the [TileSetAtlasSource] class.
 		Depending on the TileSet source type, those IDs might have restrictions on their values, this is why the base [TileSetSource] class only exposes getters for them.
 		You can iterate over all tiles exposed by a TileSetSource by first iterating over coordinates IDs using [method get_tiles_count] and [method get_tile_id], then over alternative IDs using [method get_alternative_tiles_count] and [method get_alternative_tile_id].
+		[b]Warning:[/b] [TileSetSource] can only be added to one TileSet at the same time. Calling [method TileSet.add_source] on a second [TileSet] will remove the source from the first one.
 	</description>
 	<tutorials>
 	</tutorials>

scene/resources/tile_set.cpp

@@ -479,6 +479,11 @@ int TileSet::add_source(Ref<TileSetSource> p_tile_set_source, int p_atlas_source
 	sources[new_source_id] = p_tile_set_source;
 	source_ids.push_back(new_source_id);
 	source_ids.sort();
+	TileSet *old_tileset = p_tile_set_source->get_tile_set();
+	if (old_tileset != this && old_tileset != nullptr) {
+		// If the source is already in a TileSet (might happen when duplicating), remove it from the other TileSet.
+		old_tileset->remove_source_ptr(p_tile_set_source.ptr());
+	}
 	p_tile_set_source->set_tile_set(this);
 	_compute_next_source_id();
 
@@ -504,6 +509,16 @@ void TileSet::remove_source(int p_source_id) {
 	emit_changed();
 }
 
+void TileSet::remove_source_ptr(TileSetSource *p_tile_set_source) {
+	for (const KeyValue<int, Ref<TileSetSource>> &kv : sources) {
+		if (kv.value.ptr() == p_tile_set_source) {
+			remove_source(kv.key);
+			return;
+		}
+	}
+	ERR_FAIL_MSG(vformat("Attempting to remove source from a tileset, but the tileset doesn't have it: %s", p_tile_set_source));
+}
+
 void TileSet::set_source_id(int p_source_id, int p_new_source_id) {
 	ERR_FAIL_COND(p_new_source_id < 0);
 	ERR_FAIL_COND_MSG(!sources.has(p_source_id), vformat("Cannot change TileSet atlas source ID. No tileset atlas source with id %d.", p_source_id));
@@ -3140,9 +3155,10 @@ bool TileSet::_set(const StringName &p_name, const Variant &p_value) {
 			// Create source only if it does not exists.
 			int source_id = components[1].to_int();
 
-			if (!has_source(source_id)) {
-				add_source(p_value, source_id);
+			if (has_source(source_id)) {
+				remove_source(source_id);
 			}
+			add_source(p_value, source_id);
 			return true;
 		} else if (components.size() == 2 && components[0] == "tile_proxies") {
 			ERR_FAIL_COND_V(p_value.get_type() != Variant::ARRAY, false);
@@ -3587,6 +3603,10 @@ void TileSetSource::set_tile_set(const TileSet *p_tile_set) {
 	tile_set = p_tile_set;
 }
 
+TileSet *TileSetSource::get_tile_set() const {
+	return (TileSet *)tile_set;
+}
+
 void TileSetSource::reset_state() {
 	tile_set = nullptr;
 };

scene/resources/tile_set.h

@@ -420,6 +420,7 @@ public:
 	int add_source(Ref<TileSetSource> p_tile_set_source, int p_source_id_override = -1);
 	void set_source_id(int p_source_id, int p_new_id);
 	void remove_source(int p_source_id);
+	void remove_source_ptr(TileSetSource *p_tile_set_source); // Not exposed
 	bool has_source(int p_source_id) const;
 	Ref<TileSetSource> get_source(int p_source_id) const;
 
@@ -555,6 +556,7 @@ public:
 
 	// Not exposed.
 	virtual void set_tile_set(const TileSet *p_tile_set);
+	TileSet *get_tile_set() const;
 	virtual void notify_tile_data_properties_should_change(){};
 	virtual void add_occlusion_layer(int p_index){};
 	virtual void move_occlusion_layer(int p_from_index, int p_to_pos){};