let original = [1, 1, 2, 3, 3, 4, 3, 3, 3, 5, 6, 6, 7]

var result = [Int]()

var previous = 0
original.forEach { value in
    if value != previous {
        previous = value
        result.append(value)
    }
}

print(result)

let result2 = original.reduce([]) { (array, value) -> [Int] in
    guard let last = array.last else {
        return [value]
    }

    return last == value ? array : array + [value]
}

print(result2 == result)