iTunes Library add/remove a "logic" question...

This might to difficult to explain, but I was wondering if someone could explain how iTunes would "look at" this situation, or what how it would re-act, etc...

Anyways, say I have a folder called "Podcasts"... Under this folder I have several sub-folders that are the actual names of the podcasts. And finally in each subfolder are several mp3 files with the various podcast episodes.

My question is... If I choose to "add a folder to my library" in iTunes, We will use "Podcasts" in this example. Then everytime I add a new mp3 to any of the subfolders under "Podcasts", it will show up in iTunes... Ok, thats great. Well, say I clear a song off of my Library... Yet I don't delete it off of my hard drive. So my song is still in a sub-folder of "Podcasts". How does iTunes "know" to not add that song to my library in the future, say when I open up iTunes again, or when I add more mp3s to that sub-folder? I'm hoping it won't re-add songs if I clear them from the library?

I'm guessing it has some database file with the songs you've cleared, that way it won't add them again when it autoamatically scans the folder and sub-folders for new mp3s. But I'm probably not completely right about that. Say I clear off a song called "La Isla Bonita" out of my Madonna subfolder because I find out that song has skips in it... Yet later on I delete or over write the mp3 with the exact same song (same file name) yet it doesn't have pops in it. Will iTunes know to re-add that song to my library, even though in the past I told it to "clear" it from my library?

Whew... I hope that all made since. If someone could clear this up for me I would be very greatful.

Thank you in advance,
Bryan