The concept of mirror universes and evil doppelgangers is a popular one in s.f. TV shows. The best known one is probably the Star Trek episode "Mirror Mirror" (and its sequels on Deep Space Nine). However, this episode bears a strong resemblance to the Doctor Who episode "Inferno." Talmadge, like Brigadier Lethbridge-Stewart, gets a scar on his head and becomes a sadist, the other sadist Donovan turns out to be a coward and gets shot at the end, there's a helpful young man (like Hooter) who helps the Doctor in both universes, a female associate of the Doctor's is suspicious of him but ultimately helps him (like Svetlana), and both feature a jump to an alternate universe via a power overload in a time-traveling machine.