Better error when you have duplicate map icons.

tools/icongen.cpp

@@ -214,7 +214,7 @@ void load_config(MapConfig& config, bool is_centered, std::string path, std::fun
     config.names.insert_or_assign(display, key);
 
     dbc::check(!config.colors.contains(display),
-        fmt::format("duplicate color for display={} key={}",
+        fmt::format("duplicate icon for display={} key={}",
           (int)display, (std::string)key));
 
     dbc::check(data.contains("foreground"),