The phrase "had once shown" is correct and usable in written English.
It is used to express an event that happened at an unspecified time in the past. Example Sentence: The painting that my grandmother had once shown me was one of her prized possessions.
Paul, Sr., had once shown her a porn like that.
As a boy he had once shown her the shoebox peepshow from which the novel takes its name, a miniature trompe l'oeil landscape, unexpectedly enchanting.
With the race tightening, Dr. Dean, whom the polls had once shown as the front-runner, came under a biting attack from Mr. Gephardt.
Jonathan's father had once shown him the table, and noted his son's admiration of it, but in a final note of cruelty, did not leave it to Jonathan when he died.
That she had learned about it amazed her more, for the confusion and chaos of lost patients was like the nest of rattlesnakes Verl had once shown her, a coiling mass under a shelving boulder.
Mr. Nicklinson himself had once shown athletic prowess as an avid rugby player, but a stroke in 2005 ensnared him in incurable "locked-in syndrome," which he described as a "living nightmare".
Montet-Guerin had been desperate to learn more, she said, and had once showed the ring to a psychic.
When the Guardian originally broke the story of its discovery, Simmons said his father had once showed his treasures to some academics, and was bitterly disappointed when they were dismissed as rubbish.
Recall the grace he had once showed with his left-handed hook shot.
Since the primaries, during which candidates can be forgiven some excesses, neither has once shown that he understands the threat that the budget deficit poses.
The kind of empathy that had once been shown toward borrowers when the lenders were nonprofits disappeared.
