correctly detect duplicate layers

include/llmr/style/style_parser.hpp

@@ -102,9 +102,6 @@ private:
     // This maps ids to Layer objects, with all items being at the root level.
     std::unordered_map<std::string, std::pair<JSVal, std::shared_ptr<StyleLayer>>> layers;
 
-    // Layer IDs that we have encountered previously while parsing this stylesheet.
-    std::set<std::string> existing_layer_ids;
-
     // Store a stack of layers we're parsing right now. This is to prevent reference cycles.
     std::forward_list<StyleLayer *> stack;
 

src/style/style_parser.cpp

@@ -501,7 +501,7 @@ std::shared_ptr<StyleLayer> StyleParser::createLayer(JSVal value) {
 
         const std::string layer_id = { id.GetString(), id.GetStringLength() };
 
-        if (existing_layer_ids.find(layer_id) != existing_layer_ids.end()) {
+        if (layers.find(layer_id) != layers.end()) {
             fprintf(stderr, "[WARNING] duplicate layer id %s\n", layer_id.c_str());
             return nullptr;
         }