Movie in which a frame reveals protagonist real identity?

I recently saw a movie in which in a frame the protagonist was substituted by another character. In this way the revealed the protagonist s real identity, but you catch it only if you re very lucky or careful. I remember that:
-The protagonist (or maybe wasn't the protagonist, but an important character anyway) was female

-That in the scene see was sat, then someone passes between her and the camera,and when she reappears she s the other character for just a moment and then the first character reappears.

Please this is driving me crazy!

It s not fight club or memento!

Wasn't their a scene like that in Domino.

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Nope, haven't seen that

I vaguely remember something like that in Before I Go To Sleep, which is a thriller about memory and identity.

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"Film is a disease. When it infects your bloodstream it takes over as the number one hormone. It bosses the enzymes, directs the pineal gland, plays Iago to your psyche. As with heroin, the antidote to Film is more Film." - Frank Capra

Nope it wasn't that movie either!!

Elizabeth Hurley in Bedazzled 2000 ?

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"We're all genyav. He's a collective manifestation of all our greatest hopes and fears, our most rapturous dreams, our most horrific nightmares..." - planet news

Nope