Code copied from https://github.com/json4s/json4s/blob/3.4/ast/src/main/scala/org/json4s/Merge.scala#L69

json4s-merge.scala

def mergeFields(vs1: List[JField], vs2: List[JField]): List[JField] = {
  def mergeRec(xleft: List[JField], yleft: List[JField]): List[JField] = xleft match {
    case Nil => yleft
    case (xn, xv) :: xs => yleft find (_._1 == xn) match {
      case Some(y @ (yn, yv)) =>
        JField(xn, merge(xv, yv)) :: mergeRec(xs, yleft filterNot (_ == y))
      case None => JField(xn, xv) :: mergeRec(xs, yleft)
    }
  }

  mergeRec(vs1, vs2)
}