Lists: Idiomatic way of filtering similar elements

A small improvement to the solution provided by @gil.fernandes can be made by using the API introduced in the Kotlin 1.1 standard library: .groupingBy { ... }. Instead of creating the groups right away, it creates a lazy Grouping, which can then be queried. In particular, we can ask it for .eachCount(), which returns Map<T, Int>:

val l = listOf("a", "a", "b", "c", "b", "d")

val result = l.groupingBy { it }.eachCount().filterValues { it > 1 }.keys

It is a little bit more efficient than using .groupBy { ... }, because it does not store the groups in the lists.

If you want to filter out duplicates (non distinct items) you could try:

fun main(args: Array<String>) {
    val l = listOf( "a", "a", "b", "c", "b", "d" )
    println(l.groupBy { it }.filter { it.value.size > 1 }.keys)
}

This prints out:

[a, b]

Explanation: l.groupBy { it } produces a grouped map with this content:

{a=[a, a], b=[b, b], c=[c], d=[d]}

Then a filter is applied to this map, filtering by the values (which are lists) with a length superior to 1: filter { it.value.size > 1 }.

Finally the keys are extracted with keys.