so start a run get to the part were you get the post card get to undyne and before the flag were she fights you use the post card and call toriel press x to remove the post card and move with the call text still there wean undyne attacks you press z and now there will be two undyne's fight her until the game has an error restart the game you will have debug mode. hope this helps :p (ps if it does not work try the web version)

(ps ps! talk to the 2th undyne to get a error faster)