let phrase: String = "If you know what you know if I..."
let words = phrase.split(separator: " ")

/// Check if a given word is includen in a string
/// before the own word.
func containsWord(_ word: String, offsetLimit limit: Int) -> Bool
{
    return words.enumerated()
        .filter({ $0.offset < limit })
        .map({ String($0.element) })
        .contains(word)
}

for (index, word) in words.enumerated()
{
    if containsWord(String(word), offsetLimit: index)
    {
        print("The first word duplicated is \(word)")
        break
    }
}