When I export a movie to Quicktime or FB, the movie shows in duplicate on the screen with one movie playing in the top half and an identical one playing simultaneously underneath it. I cannot resolve it.

When I export a movie to Quicktime or FB, the movie shows in duplicate on the screen with one movie playing in the top half and an identical one playing simultaneously underneath it. I cannot resolve it and cannot find a similar problem online. I followed the instruction in the Missing Manual and deleted the com.apple.imovie.plist and the com.apple.iApps.plist and restarted iMovie but still have the same problem. Any help would really be appreciated.