Hattori and his doppelgängers is Episode 145 of the 1981 Ninja Hattori-kun anime.
Hattori is in for a tight spot when Kenichi, Shinzo, Shishimaru, Yumeko and Tsubame all disguise themselves as the young Ninja. In retaliation, Hattori disguises as Kemumaki.
One night at the flat, Kemumaki has a ghostly premonition. He runs about in a deep, dark city, and finds a doppelgänger of himself! As his evil twin turns on him, Kemumaki jolts back to life. He is relieved that it was all a dream, but still shaken.
Later on at the waste ground, Hattori and the gang play Kick the Can, but Shinzo vehemently insists that they start again, as he feels Hattori kicked the can to hard. They start again. Kenichi and Yumeko run into town so as to hide. As they run, they pass a bookshop, where Kemumaki is reading a book on doppelgängers. He reads the section where one can be able to see doppelgängers without using Ninja Techniques; however the doppelgängers have many more features to themselves.
Meanwhile, at the waste ground, Hattori finishes counting. He uses his ninja techniques, and tracks Kenichi and Yumeko down, and finds them hiding near a café. They run off, but Hattori overtakes them and goes back to the waste ground. There he disguises as a rock, and finds Tsubame. He then finds Shinzo and Shishimaru with his cloth glider. Once he gets back, Kenichi expresses his disgust at Hattori using Ninja Techniques to cheat. Yumeko, Shinzo and Shishimaru all side with Kenichi, but Tsubame is all in favour for Hattori. Kenichi suggests a tricky idea...
In the attic, Kenichi, Shinzo, Yumeko, Tsubame and, believe it or not, Shishimaru, all disguise themselves as Hattori. Yumeko and Tsubame flick makeup at each other, Shinzo does a makeover on Kenichi, and Shishimaru puts on a mask of Hattori after he pulls his trousers up his stilts.
Back at the waste ground, Hattori stares incredulously at his five doppelgängers. Tsubame points out that they could all get confused, so Hattori jumps into a bush and disguises himself as Kemumaki.
At the bookshop, Kemumaki reads the last of the book, which says that if one meets his alter ego, then there will be a time paradox which may result in the end of the world. As he leaves the shop, he comes upon the waste ground, where he sees two Hattoris running to find a place to hide. As he looks on in shock and incredulity, he sees another one jump into a tree. Kemumaki, now in full shock, climbs up a common wire line, and gets his telescope out. He then sees three more doppelgängers. He assumes that the gang must be practising the Ninja Technique of disguise. He goes to the waste ground, ready to inform Hattori that he is in grave danger.
Meanwhile, Hattori (who is still disguised as Kemumaki) searches for his doppelgängers. He finds one, but doesn't know if it's Kenichi or Tsubame or Shinzo or Shishimaru or Yumeko (in reality, it's Yumeko). As Shinzo kicks the can, Yumeko expresses her relief that Hattori couldn't identify any of the gang. Just then, Kenichi gets bored and falls asleep in his pipe.
In the meantime, Kemumaki runs on, but suddenly screeches to a halt. Good Kemumaki pops up and tells Kemumaki to go ahead, but Bad Kemumaki says otherwise. Kemumaki listens to Good Kemumaki and runs on. As he reaches the waste ground, he sees two motionless legs that resemble Hattori's sticking out of the pipe. Assuming that his rival is dead, Kemumaki bursts into tears and regrets all he has done. However, it's just Kenichi, out cold in his pipe. Meanwhile, Hattori, still disgused as Kemumaki, hears the commotion, and runs to join the others. As Hattori's disguise catches Kemumaki's eye, he runs off in fear. As Hattori and the others stare incredulously, Kenichi arises from his stupor. Hattori tells the gang that he doesn't know what sort of person Kemumaki is. Back at the flat, Kagechiyo tries to comfort his distraught master, but is unsuccessful.