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Leo, a source said, "bought the bag for his mom."
In his winning bid for the 10,000-euro (about $11,300) Chanel bag, Leo outbid other entertainment industry heavyweights, including Sony Pictures Classics co-founder Tom Bernard.
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The charity auction raised funds to bring heart care to underprivileged children around the world.
Both Leo and Paris are in Cannes for the legendary Cannes Film Festival.