The way this bug is created:

1. You've sent a transaction some time ago which was included in the tx pool and stuck their for some time.
2. You've downloaded a buggy version of simplewallet this morning.
3. To fix the bug you deleted "wallet.bin" that contains all the info regarding already used inputs.
4. While refreshing, the wallet analyzed the blockchain and found all the used inputs there but it knows nothing about tx pools transactions waiting to be included into the blocks.
5. When trying to send a new transfer the wallet randomly chooses the inputs that are awaiting in the tx pool and therefore detects a double-spending attack.

Good news, it is solvable and we'll fix the wallet asap.
Bad news, it seems that you should wait up to 24 hours so that the transaction is either accepted by the network or is returned from tx pool.