They are different shoes! The first thing Jack sees when he opens his eyes is the white shoe in the tree. Nikki's shoe is a different color (green, I think). But there is definately a shoe-ish sort of theme happening on the show. The first shot we get of Locke is of his feet - shoe missing - and he wiggles his toe. Season one early on - Kate wants to go on the hike and Jack tells her she will need better shoes, then we see her removing a dead person's shoes - and we only see the boots she's removing. The Others walk around sans shoes in the jungle, even the Ben-led Others (if you think there are two sets) are in disguise and barefoot at the Pala Ferry, right before they let Michael leave. Oh yeah, and the Ring Lady in the Des epi points out the man in the red shoes. Christian walking through the jungle had on a suit and white tennis shoes. Charlie kept his heroin in his black&white van shoe.

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