l1 = List((1,"a"),(3,"b"))
l2 = List((3,"a"),(4,"c"))

l3 = List((4,"a"),(3,"b"),(4,"c"))

val l = l1 ::: l2
val m = Map[String, Int]()
(m /: l) {
  case (map, (i, s)) => { map.updated(s, i + (map.get(s) getOrElse 0))}
}.toList // Note: Tuples are reversed.

(l1 ++ l2).groupBy(_._2).mapValues(_.unzip._1.sum).toList.map(_.swap)

scala> l1 ++ l2
res0: List[(Int, java.lang.String)] = List((1,a), (3,b), (3,a), (4,c))

scala> res0.groupBy(_._2)
res1: ... = Map(c -> List((4,c)), a -> List((1,a), (3,a)), b -> List((3,b)))

scala> res1.mapValues(_.unzip)
res2: ... = Map(c -> (List(4),List(c)), a -> (List(1, 3),List(a, a)), b -> (List(3),List(b)))

scala> res1.mapValues(_.unzip._1)
res3: ... = Map(c -> List(4), a -> List(1, 3), b -> List(3))

scala> res1.mapValues(_.unzip._1.sum)
res4: ... = Map(c -> 4, a -> 4, b -> 3)

scala> res4.toList
res5: List[(java.lang.String, Int)] = List((c,4), (a,4), (b,3))

scala> res5.map(_.swap)
res6: List[(Int, java.lang.String)] = List((4,c), (4,a), (3,b))

import scalaz._
import Scalaz._

val l3 = (l1.map(_.swap).toMap |+| l2.map(_.swap).toMap) toList

for ( (k,v) <- (l1++l2).groupBy(_._2).toList ) yield ( v.map(_._1).sum, k )

val l3 = (l1 zip l2).foldRight(List[(Int, String)]()) {
  case ((firstPair @ (firstNumber, firstWord),
        secondPair @ (secondNumber, secondWord)),
        result) =>
    if (firstWord == secondWord)
      ((firstNumber + secondNumber), firstWord) :: result
    else
      firstPair :: secondPair :: result
}

val lst = l1 ++ l2
lst.map(_._2).distinct.map(i => (lst.filter(_._2 == i).map(_._1).sum, i))