Keira Knightley has warned Sienna Miller to steer clear of her ex-love Jamie Dornan - in case he breaks her heart.The stunning actress - who split from Dornan last year amid claims he was jealous of her career - reportedly called Sienna after hearing she had a crush on the former model.Keira - who is now romancing 'Pride And Prejudice' co-star Rupert Friend - allegedly told Sienna the Irish hunk wouldn't be able to handle her success.A source is quoted in Britain's Daily Star newspaper as saying: "Keira went through hell with Jamie, because he couldn't deal with her fame."She's dating Rupert Friend now. But she was horrified when she heard Sienna has enjoyed a series of dates with Jamie. She likes Sienna and doesn't want to see her make the same mistakes."But stunning Sienna - who has also been linked to 'Star Wars' hunk Hayden Christensen since splitting from former fiance Jude Law - insisted she could make up her own mind.The 'Layer Cake' beauty - who struck up a friendship with Dornan after they became neighbours in London's Notting Hill - reportedly said: "I'm a big girl and I'll make my own mistakes."

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