once a something, always a something
once a (something), always a (something)
A person's innate or fundamental nature is not something they are willing or able to change. He says he's trying to leave his life of crime behind, but once a criminal, always a criminal. Even years after leaving the stage, she still can't resist performing—once an actor, always an actor.
once a —, always a —a person cannot change their fundamental nature.
1993 Margaret Atwood The Robber Bride She was once a Catholic…and once a Catholic, always a Catholic, according to her mother.