Can you explain what "duplicated images" means in more detail? It does not sound like a failure that graphics hardware or LCD would sustain.
If you can used it in some capacity, go to System Preferences Universal Access pane Seeing tab, and make sure it is not one of those settings that got enabled by mistake.
If you can, also enable "Screen Sharing" in System Preferences Sharing pane. Then, from that other Mac that is connected via Ethernet, you may be able to "remote control" the Mac with the display issue and do some more troubleshooting. To do this, on the "other" Mac, open a Finder window. Find the iMac under SHARED (in the sidebar), and select it. Over to the right, click the Share Screen button (and log in). If it's a hardware issue, the iMac's screen should appear normal when viewed from the other Mac using Screen Sharing. If it still looks messed up, then it's not a hardware issue but something related to software or settings.
Another way to confim it's a hardware issue is to start up from your Mac OS X installation disc. Insert disc and restart with the C key held down. If it's a software or settings issue, it should look normal when starting up from a different system. If it still looks messed up, then it is probably a hardware issue.