Cover the substitutions of a preset held back for its parent

A preset whose parent is not in the collection yet is read once per pass it
waits, and each read appends to the caller's substitutions list. The load drops
what it read before deferring, so only the pass that keeps the preset reports
its substitutions; without that, one preset is listed once per pass and the
count depends on how deep it sits in the hierarchy.

tests/libslic3r/test_preset_bundle_loading.cpp

@@ -448,6 +448,31 @@ TEST_CASE("Presets inheriting each other in a cycle are reported, not loaded", "
     CHECK(bundle.has_errors());
 }
 
+TEST_CASE("A preset held back for its parent reports its substitutions once", "[Preset][Inherits][Regression]")
+{
+    ScopedTemporaryDir temp_dir;
+    PresetBundle       bundle;
+
+    // The child sorts before its parent, so it is held back for a pass and its file is read
+    // twice. The bogus boolean makes every read produce a substitution.
+    const fs::path preset_dir = temp_dir.path() / PRESET_PRINT_NAME;
+    fs::create_directories(preset_dir);
+    std::ofstream((preset_dir / "AA Child.json").string())
+        << R"({"type":"process","name":"AA Child","from":"User","version":"1.0.0",)"
+        << R"("inherits":"ZZ Root","spiral_mode":"sometimes"})";
+    write_sparse_preset(preset_dir / "ZZ Root.json", "ZZ Root", "", {{"layer_height", "0.3"}});
+
+    PresetsConfigSubstitutions substitutions;
+    bundle.prints.load_presets(temp_dir.path().string(), PRESET_PRINT_NAME, substitutions,
+                               ForwardCompatibilitySubstitutionRule::Enable);
+
+    REQUIRE(bundle.prints.find_preset("AA Child") != nullptr);
+    // A read that ends in the preset being held back must not leave its substitutions behind,
+    // otherwise the same preset is listed once per pass it waited.
+    CHECK(std::count_if(substitutions.begin(), substitutions.end(),
+                        [](const PresetConfigSubstitutions &s) { return s.preset_name == "AA Child"; }) == 1);
+}
+
 TEST_CASE("Renamed printer/process names are normalized into compatible lists on load", "[Preset][Rename]")
 {
     PresetBundle bundle;