How do I stop iTunes from duplicating video files?

For some reason unknown to me, iTunes duplicates the video files when they are added to the library.

For music, when I add these to the library, the files stay on the external drive, and they are listed in the library.

However, I now moved my video files to an extrnal drive, but when I want to add them back into my library, it wants to copy all of them (the full file) from the external drive to my itunes folder. I moved them off of the computer hard drive because they take up too much space. I don't want them here, but I do want them in the library so I can select and sync them.

How do I add these videos to my itunes library, but keep the actual files on the extrnal drive?