Ensure that generated import IDs are unique

editor/import/scene_import_settings.cpp

@@ -137,24 +137,23 @@ void SceneImportSettings::_fill_material(Tree *p_tree, const Ref<Material> &p_ma
 	String import_id;
 	bool has_import_id = false;
 
-	bool created = false;
-	if (!material_set.has(p_material)) {
-		material_set.insert(p_material);
-		created = true;
-	}
-
 	if (p_material->has_meta("import_id")) {
 		import_id = p_material->get_meta("import_id");
 		has_import_id = true;
 	} else if (!p_material->get_name().is_empty()) {
 		import_id = p_material->get_name();
 		has_import_id = true;
+	} else if (unnamed_material_name_map.has(p_material)) {
+		import_id = unnamed_material_name_map[p_material];
 	} else {
-		import_id = "@MATERIAL:" + itos(material_set.size() - 1);
+		import_id = "@MATERIAL:" + itos(material_map.size());
+		unnamed_material_name_map[p_material] = import_id;
 	}
 
+	bool created = false;
 	if (!material_map.has(import_id)) {
 		MaterialData md;
+		created = true;
 		md.has_import_id = has_import_id;
 		md.material = p_material;
 
@@ -164,6 +163,7 @@ void SceneImportSettings::_fill_material(Tree *p_tree, const Ref<Material> &p_ma
 	}
 
 	MaterialData &material_data = material_map[import_id];
+	ERR_FAIL_COND(p_material != material_data.material);
 
 	Ref<Texture2D> icon = get_theme_icon(SNAME("StandardMaterial3D"), SNAME("EditorIcons"));
 
@@ -553,9 +553,9 @@ void SceneImportSettings::open_settings(const String &p_path, bool p_for_animati
 
 	base_path = p_path;
 
-	material_set.clear();
 	mesh_set.clear();
 	material_map.clear();
+	unnamed_material_name_map.clear();
 	mesh_map.clear();
 	node_map.clear();
 	defaults.clear();

editor/import/scene_import_settings.h

@@ -107,6 +107,7 @@ class SceneImportSettings : public ConfirmationDialog {
 		HashMap<StringName, Variant> settings;
 	};
 	HashMap<String, MaterialData> material_map;
+	HashMap<Ref<Material>, String> unnamed_material_name_map;
 
 	struct MeshData {
 		bool has_import_id;
@@ -141,7 +142,6 @@ class SceneImportSettings : public ConfirmationDialog {
 	void _fill_scene(Node *p_node, TreeItem *p_parent_item);
 
 	HashSet<Ref<Mesh>> mesh_set;
-	HashSet<Ref<Material>> material_set;
 
 	String selected_type;
 	String selected_id;