Q: Why Are My Songs Listed Twice?

Did you copy the entire library across, or just the media folder and did you then add media to the library? iTunes won't play WMA files so any added to the library are converted according to your current import preferences. If you use iTunes own feature to downsample tracks for loading onto the iPod it will be creating AAC versions. I believe they are created on a temporary basis and moved to the device rather than as parallel duplicates in the media folder, but it isn't a feature I've used or tested.

tt2

You may have either left something behind, or nor copied the iTunes folder from <User's Music> to <User's Music> but placed it at some other location. When done properly iTunes starts up on the new computer looking exactly like it did on the older one, there is no prompt to find media. Take a look at the description of the iTunes folders in Make a split library portable. In some cases it is possible that on the first run iTunes has replaced the original library with an empty one in the manner shown in Empty/corrupt iTunes library after upgrade/crash in which case the same techniques might be able to restore the previous library. Alternatively you could redo the migration.

tt2

Apple's official advice on duplicates is here at the link you've posted: Find and remove duplicate items in your iTunes library. It is a manual process and the article fails to explain some of the potential pitfalls such as lost ratings and playlist membership, or that sometimes the same file can be represented by multiple entries in the library and that deleting one and recycling the file will break the other.

Use Shift > View > Show Exact Duplicate Items to display duplicates 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, however this works best when performed immediately after the dupes have been created.  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, this post for detailed instructions, 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.)

The most recent version of the script can tidy dead links as long as there is at least one live duplicate to merge stats and playlist membership to and should cope sensibly when the same file has been added via multiple paths.

You should probably move all of the WMA files out to some archive location so they cannot be accidentally reimported & converted again.

tt2