Convert List of tuple to map (and deal with duplicate key ?)

Group and then project:

scala
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 val x = List("a" -> "b", "c" -> "d", "a" -> "f")

//x: List[(java.lang.String, java.lang.String)] = List((a,b), (c,d), (a,f))

scala
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 x.groupBy(_._1).map { case (k,v) => (k,v.map(_._2))}

//res1: scala.collection.immutable.Map[java.lang.String,List[java.lang.String]] = Map(c -> List(d), a -> List(b, f))

More scalish way to use fold, in the way like there (skip map f step).

Here's another alternative:

x.groupBy(_._1).mapValues(_.map(_._2))