Doctor Double X is an enemy of Batman.
Simon Ecks developed a means by which to create an energy twin that could commit criminal acts independent of its host form. To distinguish between the two, "Doctor X" wore a costume with a single X emblem upon his chest, while "Doctor Double X" (appropriately enough) wore a costume with two letters stitched upon his chest. Doctor Double X first faced Batman and his teenage partner Robin many years ago and was able to distract them by making them believe that they only faced one opponent, when in fact, they were facing two. Ultimately, the Dynamic Duo were able to capture Doctor X and send him to prison, but he was freed soon after by Doctor Double X.
Free from prison, Doctor Double X attempted to bait Batman into a trap, but Batman was ready for him. Deciding to give the Doctor a taste of his own medicine, Batman allowed a Batman robot to take his place as Double X's victim. While the Doctor believed that he had captured Batman, the real Batman emerged and turned the tables on him. Doctor Double X was apprehended and taken to the Happydale Sanitarium. During this time, Simon Ecks lost his memory of his scientific and criminal career.
Years later, Ecks regained his memory and developed another duplicator device that enabled him to escape from the Sanitarium. He matched wits once again with Batman, although this time, Batman was able to defeat him by building his own machine that generated an energy duplicate of himself.
Doctor Double X was not seen again for many years. When he emerged a third time, he was defeated by the combined efforts of Batman and Superman.
In the show Batman the Brave and the Bold, Doctor Double X appears in the episode "A Bat Divided". He tries to start a nuclear meltdown to increase his doppelgänger's power. However the blast results in Batman splitting up and the creation of the hero Firestorm. He later kidnappes Firestorm and put him in a machine to transfer his power to his doppelgänger. However the Batmen frees Firestorm and they defeat Double X. After that his machine is used to return Batman. He later makes a cameo in the episode "Joker the Vile and the Villainess" in a bar where villains hang out. In this show he is voiced by Ron Perlman.