VirtualBox: Error: Cannot register the hard disk

If when cloning VirtualBox VMs you get an error like this:

$ VBoxManage clonehd WinXP-IE7.vdi WinXP-IE8.vdi 
VirtualBox Command Line Management Interface Version 2.2.4
(C) 2005-2009 Sun Microsystems, Inc.
All rights reserved.

ERROR: Cannot register the hard disk '/home/scol/.VirtualBox/VDI/WinXP-IE7.vdi' with UUID {3858b1b1-c306-4505-8264-235af812f337} because a hard disk '/home/scol/.VirtualBox/VDI/WinXP-IE7.vdi' with UUID {3858b1b1-c306-4505-8264-235af812f337} already exists in the media registry ('/home/scol/.VirtualBox/VirtualBox.xml')
Details: code NS_ERROR_INVALID_ARG (0x80070057), component VirtualBox, interface IVirtualBox, callee nsISupports
Context: "OpenHardDisk(Bstr(szFilenameAbs), AccessMode_ReadWrite, srcDisk.asOutParam())" at line 603 of file VBoxManageDisk.cpp

Talk about a completely ambiguous error message! The solution is to provide an absolute path to the source VDI and target VDIs (leaving the path off the target will put the image in ~/.VirtualBox/HardDisks by default):

$ VBoxManage clonehd $(pwd)/WinXP-IE7.vdi $(pwd)/WinXP-IE8.vdi 
VirtualBox Command Line Management Interface Version 2.2.4
(C) 2005-2009 Sun Microsystems, Inc.
All rights reserved.

0%...10%...20%...30%...40%...50%...60%...70%...80%...90%...100%
Clone hard disk created in format 'VDI'. UUID: b7166421-9743-4808-aedf-3c90aeb7c902