The "send me your money and I'll send double back" scam is a staple among kids playing MMOs since at least the days of Runescape. Doubtless it goes a lot further back.
How does this work exactly? I can see how it works in the short term e.g. you give me money, I give you double, you give me lots of money and I disappear with it. But it seems he had an ongoing relationship with the Daubi bank manager, how is that possible?
Going back to MMOs, scammers in EVE have a jumble of variants on this scam. Typically they'll double your money if it's below a certain amount, but run away with any larger sums. Sometimes they frame the game as a lottery or game of skill: only the first person whose cash transfer shows up in their wallet will have their money doubled. With arbitrary rules like that, it's easier to give excuses as to why they weren't able to double the money. Oh, you didn't read the fine print in my bio: better luck next time! A good rube might fork over multiple donations before he realizes his money won't be coming back. Of course, scamming in that game is perfectly legal, so it's a lot easier to just block and ignore your marks.