Determining Which Tracks Are Duplicates
The screen should look something like second the image below. Generally, iTunes will list tracks with the same title and artist as duplicates. This is where your work in removing duplicates from your iTunes library comes in. Carefully check each track, making sure that it is in fact a duplicate. You'll want to separate the true copies from instances where you have multiple versions of the same song.
As an example, let's take a look in image three below. You'll notice that the two tracks listed share the same title and artist. But look closely at these tracks. The second track has a significantly longer run time than the first. Both songs also show different years of release, and appear on different albums. In this instance, we are not looking at duplicate tracks but rather two versions of the same song. We will not want to delete either of those.
Take a look at image four, however. In this case, both tracks have the same runtime, release year, and album. The first track was added a couple of years after the second, though. Generally speaking, you'll probably want to keep the newer version of any duplicate track. If you aren't sure, go ahead and right-click on each track and select "Get Info" from the menu. From here, you can get detailed information about each track so you can determine which tracks to keep.