The following:

bitcoind setaccount <unknown_address> label 

adds a new sending address in the Address Book of the GUI.

Using:

bitcoind setaccount <known_sending_address> label 

will rename the sending address listed in the Address Book.
This is fine, but, in the same time, it will also pop up a new receiving address labeled with the old name.
This behavior is quite confusing and I think it is not expected.