If you were on Newgrounds and stumbled onto this Flash game during Halloween 2008, you'd understand this. It's Tails VS Tails Doll within the former's mindscape!
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TheBlox (not sure if he's still around the site) is known for making Tails' Nightmare, a Halloween-themed tech demo for his then-in development game, Cosmic Rush. In it, the player controls Sonic's trusty twin-tailed fox sidekick Tails as he navigates his way through the dreaded dreamscape, culiminating in a battle against Tails Doll, his robotic counterpart that is essentially supernatural.

A sequel came out that took place within an underground tunnel and with pools of acid. There was meant to be a third game set in a volcanic landscape with Sonic.exe as one of the bosses, but ultimately the game was shelved due to the eventual shutdown of Flash. Such a darn shame. Oh well, here's to the legacy of the titular fox's own Flash game duology (I would say trilogy but y'know).

Let me know if you got a chance to play Tails' Nightmare 1 & 2 as well (and maybe even the third game's demo).
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Sonic the Hedgehog© Sega