Thanks for not telling us something about your computer and operating system. Guesswork and mysteries are so much fun...
Let me guess... You cloned the hard disk and are now wondering why it's complain about duplication. Duh.
If you happen to have XP or W2K, right click on the "My Computah" icon and select "Properties". Select the "Network Identification" tab. Change the system name. Reboot.
If you happen to have W98 or WinME, right click on the the "Network Neighborhood" icon and select "Properties". "Select the "Identification" tab. Change the computah name.
If you did cloned a W2K or XP system, you're about to have a different problem. Windoze distinguishes its network computers by assigning a globally unique system ID code to each machine. It's considered bad form to have two identical machines with identical SID codes. Start reading here: