Just saw this 1959 movie on TCM.
Cast includes Millicent Martin (aka Ma Moon on "FRASIER") in a small
role at the start of the picture. Robert Shaw also has a bit part as
a paparazzo.
The film has never been released on video or DVD, though I remember
seeing it decades ago on TV.
Dated but well acted, with Dirk Bogarde and Olivia de Havilland in the
main roles. While we associate these two with different movie eras,
in fact Bogarde was only five years younger than de Havilland.
An tip-off to whether or not Bogarde's main character may be an
imposter is the hair in the flashback scenes.
Another movie with a parallel premise/plot, made a couple of years
later, was "The Scapegoat" with Alec Guinness in a double role and
Bette Davis (a crony of de Havilland's) as a morphine-addicted
mother. Not sure this has ever been on TV --- I saw when it was
released circa 1960.
CPJ