Cleaning up iTunes library to avoid duplicates

I hate duplicate music files when using UPnP. It happens because I bought some music in iTunes as .m4a and need to convert it to .mp3 to use it on different players. So a cleanup of my library was neccessary, but how to do it?

Clean your iTunes libray

1. Identify m4a files
2. Convert it to mp3 (with iTunes)
3. move m4a to another location outside iTunes library
4. remove invalid references in iTunes (because of the move)

Here is how to do on a Win 7 machine:

1. Use terminal to list files in a text file:
   - Navigate to your music folder and then
   for /r %M in (*.m4a) do @echo "%~nxM", "%~zM", "%~dpM" >>filelist.txt
   (http://superuser.com/a/458184/291367)
2. Identify files to convert and convert it in íTunes to mp3
3. Move .m4a (terminal, first navigate to your music folder and then)
   forfiles /S /M *.m4a /C "cmd /c move @file C:\TARGET"
   (http://superuser.com/a/458178/291367)
4. see http://paulmayne.org/blog/2007/11/how-to-remove-broken-or-dead-tracks-from-itunes/ how to do this