Identify unique elements within a Swift Array, preserving original order

Unique Elements.swift

// Order does matter
extension Array where Element: Hashable {
    var uniqueElements: [Element] {
        var seen = Array<Element>()
        var underlyingIterator = makeIterator()

        while let next = underlyingIterator.next() {
            if !seen.contains(next) {
                seen.append(next)
            }
        }
        return seen
    }
}

// Usage:
[1,1,2,2,3,3].uniqueElements // Results in [1,2,3]