The other question is; "Where did the duplicates come from?"
Could the m4a files have been created from the wma files when you synchronised music with an iDevice (iPhone or iPad)?
Typically an iDevice can't play wma files, so in synchronising to such a device MC will convert them to a suitable format. In this case m4a. Could you have accidently or deliberately synchronised some files back to your hard disk, into a location that MC also imports?
If this was how the files were created, and you don't really want them, then delete them, but remember to delete from disk as well as from the library. You could use a simple Smartlist to find them all, or if there are only a few and you know which Albums are affected, you could just search all audio files for m4a, select and delete them.
Just be careful not to delete what you want to keep!
PS: Look at the files "Date Created" in the Tag window, or by adding "Date Created" as a column in the view you use, to see which file type was created first, and if all the m4a files were created on the same day and time, indicating perhaps s synchronisation process you ran.
