More proper fix for #3484.

folder_paths.py

@@ -258,7 +258,7 @@ def get_save_image_path(filename_prefix, output_dir, image_width=0, image_height
         raise Exception(err)
 
     try:
-        counter = max(filter(lambda a: a[1][:-1] == filename and a[1][-1] == "_", map(map_filename, os.listdir(full_output_folder))))[0] + 1
+        counter = max(filter(lambda a: os.path.normcase(a[1][:-1]) == os.path.normcase(filename) and a[1][-1] == "_", map(map_filename, os.listdir(full_output_folder))))[0] + 1
     except ValueError:
         counter = 1
     except FileNotFoundError: