>That was solved by David Chaum over a decade before Bitcoin.
To be exact, David Chaum figured out a system that prevents double spending yet doesn't have traceable transactions. But it still required a central authority that keeps the ledger and which can block your account.
The innovation of Bitcoin is to prevent double spending without having any centralization at all.
Bitcoin's innovation was allowing currency to be issued without a central authority and still preserving the security against double spending. That comes at a high cost: you lose secure offline transactions or you incur a scalability problem (for Bitcoin, it is the former: no offline transactions). Chaum proved that this is the case for any digital cash system, regardless of central authorities (and in fact, central authorities are a handy way to deal with the issue: you choose the scalability hit and then allow the central authority to exchange used currency units for fresh ones):
Sounds cool, where can I download the source code and test this system?
It was not exactly something you could compile and run yourself. On the other hand, if you have time, you can go ahead and implement any of the systems Chaum proposed (you'll probably want to scroll down to the 80s and 90s):
Really though, Chaum's digital cash ideas involved building infrastructure around existing currency to protect privacy and prevent certain kinds of fraud (e.g. double spending, identity theft). Chaum was not trying to bring down the world's economic system with digital cash, he was just trying to give people a more secure way to pay for things electronically. The failure of his system in the real world was likely linked to the strong pushback by the US government against the use of good cryptography by "commoners."