Who's This? The doofy Batman villain on page 25 of Who's Who vol. VI.
How you could have heard of him: He's had small appearances in Young Justice (the 90s comic) and one pretty funny appearance at a convention in Titans #17 selling Deathstroke masks, but you probably saw him on an episode of Batman: The Brave and the Bold ("A Bat Divided" feat. Firestorm).
Example story: Detective Comics #316 (1963)
So what happened to Double-Batman? In the issue's saddest moment, he commits suicide.
Who else? In Volume VI, the only other contender might have been the Demolition Team, maybe Dr. Phosphorus or Deathbolt, but I'll just have to get them on the next go-round. On to Volume VII!