diff --git a/geojson.cpp b/geojson.cpp
index 7aae06a1..1dbea6aa 100644
--- a/geojson.cpp
+++ b/geojson.cpp
@@ -431,10 +431,14 @@ int serialize_geometry(json_object *geometry, json_object *properties, json_obje
 
 			if (tas.type >= 0) {
 				auto fk = layermap->find(layername);
-				fk->second.file_keys.insert(std::pair<std::string, type_and_string_stats>(tas.string, type_and_string_stats()));
+
+				auto fka = fk->second.file_keys.find(tas.string);
+				if (fka == fk->second.file_keys.end()) {
+					fk->second.file_keys.insert(std::pair<std::string, type_and_string_stats>(tas.string, type_and_string_stats()));
+					fka = fk->second.file_keys.find(tas.string);
+				}
 
 				if (track) {
-					auto fka = fk->second.file_keys.find(tas.string);
 					if (fka == fk->second.file_keys.end()) {
 						fprintf(stderr, "Can't happen (tilestats)\n");
 						exit(EXIT_FAILURE);