What good is being able to import a key if you can't import one that's been used before? Is this a typo? Is this a joke? What good is backing up my key pairs for a rainy day if I won't be allowed to import them if they've been used before? This can't be right, is it?
(space)bitcoind importprivkey "5yourveryveryveryverylongprivatekeystring" "my-new-key" # "my-new-key" is a label for the key/address pair and is optional
The importing process is now started. Bitcoind will rescan the entire block data to ensure this key has not been used before. This process will take from one to two minutes, depending on your CPU performance. DO NOT abort it before finishing!
If no errors occurs, the import is a success and Bitcoin-QT users will be able to see the new address in the GUI immediately. If you need to import more keys, just repeat the instructions above. There is currently no command to import a batch of private keys so you will need to wait a minute or two for each key to be imported.
https://en.bitcoin.it/wiki/How_to_import_private_keys
