This might have come up before, but even though Bart Kepple isn't the only COLUMBO character to kill his own blackmail victim instead of getting killed BY his blackmail victim (there's also Paul Gerard and Brimmer), he must be the only one to do that, then get a blackmailer himself, then kill THAT person.
In fact, is there any other murder story (in or out of COLUMBO) where that happens?
Not sure if this answers the question, but in "Murder By the Book", Ken Franklin kills his partner, Jim Ferris, and then kills his blackmailer, Mrs. LaSanka.