What file format are you using? I had this problem with .flv files and sorted it out by converting them to .mp4.
Lesson
Video player appears duplicated on answer screen
གྲོས་བསྟུན་འདི་བསྡམས་ཡོདཔ་ལས་ ལན་གསལ་བརྐྱབ་མི་ཚུགས་
What file format are you using? I had this problem with .flv files and sorted it out by converting them to .mp4.