What is the name of Sonic's robotic double?

Sonic's robotic double is Metal Sonic. Created by Dr. Eggman, Metal Sonic was designed to resemble Sonic and defeat him by matching or surpassing his famous speed.

Metal Sonic first appeared in Sonic CD in 1993, where he kidnaps Amy Rose and challenges Sonic to a race through Stardust Speedway. Unlike ordinary Badniks, Metal Sonic is a highly advanced antagonist with a distinctive personality and a direct rivalry with Sonic. His blue metal body, red eyes, and mechanical version of Sonic's silhouette make the “double” description especially fitting.

Metal Sonic is often confused with Shadow, Silver, or Mephiles. Shadow is an organic hedgehog-like character, Silver is a psychic hedgehog from the future, and Mephiles is a dark, non-robotic entity. Metal Sonic remains one of Eggman's most persistent creations, appearing in games such as Sonic Heroes, Sonic Generations, and Sonic the Hedgehog 4: Episode II.