Shuffle/shuffled functions & Array extensions

shuffle.swift

// (c) 2014 Nate Cook, licensed under the MIT license
//
// Fisher-Yates shuffle as top-level functions and array extension methods

/// Shuffle the elements of `list`.
func shuffle<C: MutableCollectionType where C.Index == Int>(inout list: C) {
    let c = count(list)
    for i in 0..<(c - 1) {
        let j = Int(arc4random_uniform(UInt32(c - i))) + i
        swap(&list[i], &list[j])
    }
}

/// Return a collection containing the shuffled elements of `list`.
func shuffled<C: MutableCollectionType where C.Index == Int>(var list: C) -> C {
    shuffle(&list)
    return list
}

extension Array {
    /// Shuffle the elements of `self` in-place.
    mutating func shuffle() {
        for i in 0..<(count - 1) {
            let j = Int(arc4random_uniform(UInt32(count - i))) + i
            swap(&self[i], &self[j])
        }
    }

    /// Return a copy of `self` with its elements shuffled
    func shuffled() -> [T] {
        var list = self
        list.shuffle()
        return list
    }
}

// Example of global functions
var strings = ["one", "two", "three", "four", "five", "six"]

shuffle(&strings)
// e.g., strings == [four, six, three, two, five, one]

let shuffledStrings = shuffled(strings)
// e.g., shuffledStrings == [six, four, one, five, three, two]

// Example of array methods
var numbers = [1, 2, 3, 4, 5, 6, 7, 8]

numbers.shuffle()
// e.g., numbers == [6, 1, 8, 3, 2, 4, 7, 5]

let shuffledNumbers = numbers.shuffled()
// e.g., mixedup == [4, 2, 6, 8, 7, 3, 5, 1]

this repeats elements, how can I prevent this? thank you