Depending on where you acquired your media, it's possible (likely, even) that some of your media will be misfiled. Some podcasts and audiobooks may get mixed in with your music files. TV Shows and music videos may appear in the Movies window. Look through your media and refile it. The proper way to do this is to select a file or group of files and press Command-I to bring up the Info window. Within this window click the Options tab, select the kind of media you're refiling from the Media Kind pop-up menu—TV Show, for example—and click OK. iTunes will properly tag the media and move it to the correct folder within the iTunes Source list.
You may be lucky enough to have completely different media on every Mac you own. I'm not. Once I pulled together this central library I found that I had duplicates everywhere. Before I created my "real" library, I needed to do something with the duplicate cruft.
Movies and TV shows I could deal with by simply scanning through the list and deleting duplicates as I found them—my video library is small enough that this isn't terribly wearing. However, music was a completely different matter. I had duplicates and even triplicates of some tracks.
iTunes includes a Display Duplicates command that isn't, in its default form, terribly helpful. Choose File -> Display Duplicates and it's likely you'll find studio tracks mixed in with their live and out-take counterparts. However, if you hold down the Option key, you find that this command changes to Find Exact Duplicates. That's closer to the mark, but some of these duplicates aren't exact. For example, you might find that one version is an MP3 file and an AAC-encoded version of the same track. Audiobook files are often identified as duplicates when they aren't.
If you don't have a lot of duplicates, you can cull the files by hand—selecting those files you don't want, pressing the Delete key, and choosing to toss them out. But I had a load of duplicates and iTunes wasn't entirely correct when identifying them.