// Swift 3

extension Array {

  func filterDuplicates(includeElement: @escaping (_ lhs: Element, _ rhs: Element) -> Bool) -> [Element] {

    var results = [Element]()

    forEach { (element) in

      let existingElements = results.filter {
        return includeElement(element, $0)
      }

      if existingElements.count == 0 {
        results.append(element)
      }
    }
    return results
  }
}