Why do I have duplicate songs and how do I get these down to one?

Highlight a song. Press command+r to reveal the song in Finder. Do that for the duplicate. Are they in the same location? You can also check this by getting info on a file and checking its location in the summary info, then check for the other file.

If you have a file in a folder that itunes doesn't control and then add the file to itunes it will place a cpy in the itunes folder when you add the file, leaving the original. Not sure how you ended up with two in itunes but I'd need to know the details of your preference settings.

I don't know about the rest. I have never bought music from ITMS.

Make sure they are really duplicates before you delete them. They might be another version of the same song. They might be the same song on a different album.

If you always use the same iTunes Store Account, you'll get a warning if you try to buy the same song again.

M4P = Apple protected AAC file. Probably purchased when Apple was using Digital Rights Management on regular file purchases.

You apparently also have some mp3 files from some source that are duplicates of your AAC versions.