"has once shown" is correct and usable in written English.
You can use it when you want to emphasize that something previously happened or was demonstrated. For example: "His dedication to improving his work ethic has once shown through his early starts and late finishes at the office."
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.
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".
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.
Instead, he focused on being a role model who would pave the way, showing others — as his own African-American mentors at Harvard University and Johns Hopkins University had once shown him — that it was indeed possible, and that his experience positioned him to make a unique contribution to the academic medical community.
She had once shown 11-year-old Janina her carefully-stored wedding outfit - a satin skirt and white blouse.
Foskey told his mother that a fellow inmate had once shown up at the infirmary vomiting and spitting up blood.
On Wednesday, Barakat's brother, Farris, told ABC News that Hicks had previously confronted his brother about parking in the wrong space and had once shown up to the couple's house with a gun on his belt.
