This episode is one of my favorites, even though the plot is actually a bit weak - it relies too much on the unlikelihood of the warring twins co-operating. But in Columbo, it's all about the dialog; those conversations where Columbo toys, catlike, with his suspect. In Double Shock the twin angle adds a twist to the usual dialog, as the twins are constantly pointing the finger at each other. Martin Landau is an excellent guest star, and he manages to differentiate the twin brothers quite well, especially in the scene where Dexter imitates Norman.
The supporting cast is rather strong: Tim O'Connor as the lawyer was a bit of a surprise, in his rather unethical conduct being prepared to cover up Clifford's will. Jeanette Nolan is quite fun, she unwittingly provides the crucial evidence in the case (due to the interruption of her TV program), and her animosity towards Columbo allows him to show a different side to his character, as he tries to patch up their relationship and shows a more sensitive side.
Just one more thing... How does Columbo ever get a conviction? Seriously, at the end of this episode, what evidence does he actually have against the brothers? Admittedly he proved they could have done it, but where is the evidence that they actually did? Of course, the mystery plot is not quite the point in Columbo, but still the weakness of the resolution detracts from this epsiode, when compared to other season 2 epsiodes like Requiem for a Falling Star and The Greenhouse Jungle.