Fix duplicate video files with tag: "extended-description-mixed"

[dakdouky]

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[coveralls]

Coverage decreased (-0.04%) to 73.558% when pulling e94fa00 on MOHAMEDELDAKDOUKY:master into 10ba6b8 on coursera-dl:master.

[coveralls]

Coverage decreased (-0.04%) to 73.558% when pulling e94fa00 on MOHAMEDELDAKDOUKY:master into 10ba6b8 on coursera-dl:master.