Welcome to Apple Support Communities! I'd take a look at the following article, and see if that'll help you resolve the issue.
If you're using iTunes 11, the location of this menu option isn't under File anymore, it is under View.
Have a good one,
When deduping use Shift > View > Show Exact Duplicate Items as this is normally a more useful selection. You need to manually select all but one of each group to remove. Sorting the list by Date Added may make it easier to select the appropriate tracks. If you have multiple entries in iTunes connected to the same file on the hard drive then don't send to the recycle bin. Use my DeDuper script if you're not sure, don't want to do it by hand, or want to preserve ratings, play counts and playlist membership. See this thread for background and please take note of the warning to backup your library before deduping.
(If you don't see the menu bar press ALT to show it temporarily or CTRL+B to keep it displayed)
If your aim is to move the entire library to the new computer and then see this migrate iTunes library post, otherwise iTunes will want to erase and reload your devices.