A funny thing started happening shortly after the introduction of iTunes 11 a few months ago. I began getting reports from Correspondents about usually reliable scripts that weren't working correctly. After closer scrutiny, we were able to determine that some of their media files were appearing twice in the library. That is, they were finding pairs of identical tracks in the "Music" library that each pointed to the same file. Like this:
Two "Music" track entries, same file.
This is not ever supposed to happen. iTunes should not allow a file in a unique location to be added to its library more than once.
Now, this is not the same effect as copying a library track two or more times to a playlist, whereby only one library track is involved. Nor is it the same as when iTunes appends a number to a filename to distinguish it from an earlier-added same-named file. Nor do these tracks seem to be a flavor of iTunes Match duplicates (iTunes Match may or may not be a factor in their appearance). This issue manifests as sets of two distinct library track entries, each having different IDs and both having the same value for the Location property.
I don't know how this happens. But it definitely has something to do with moving media files around the wrong way and doing so with iTunes 11. It may be a bug and it may even be fixed by now. But I don't know. I only got it to happen once myself while I was experimenting convolutedly with a different issue and I haven't been able to replicate it since.
Regardless of how these pairs came to be, how now to find them and then to deal with them? If you have a very large library, you may not even notice if you have any faux pairs.
My duplicate removal apps, Dupin and Dupin Lite, can detect this condition but won't attempt to delete any files. If the file of one of the tracks were deleted, well, then neither track would be pointing to a file any longer and you'd have a lot of dead tracks. There's a trick to using Dupin to delete an arbitrary one of each of the track pairs, but, technically, they're not the sort of "dupes" that Dupin wants to handle.
So, the applet Scan For Double Entries simply looks through your library for track entries that have the same file path (that is, the same file location) and will copy these pairs of tracks to a discrete playlist so you can sort them out yourself. Presumably you'll want to delete one of the tracks in each faux pair and I'm not sure that's something you want an automated tool to do for you, you know, automatically.
More information and download is here.