File overwrite issue when backing up iTunes

[mwchristie]

I back up my iTunes movies and TV shows by coping them to an external drive. All of a sudden I am suffering a problem - the system appears to be changing the name of the MP4 files so that it thinks they are all the same file and asks me if I want to overwrite the earlier version. I end up with one file called download on my external drive after copy/pasting for example 10 episodes of a TV show. Why is this happening? I am using Windows 7, the latest version of iTunes, a Western Digital external drive, Intel Core i7 975, 6 Gb of RAM, 64-bit operating system. Thanks for any advice you may have.