Kotlin extensions to find duplicates in an Iterable

Duplicates.kt

/**
 * Gets a set of duplicates in the given iterable.
 */
fun <T> Iterable<T>.duplicates(): Set<T> = duplicatesBy { it }

/**
 * Gets a set of duplicates in the given iterable using the specified selector.
 */
fun <T, K> Iterable<T>.duplicatesBy(selector: (T) -> K): Set<K> =
    groupingBy(selector).eachCount().filterValues { it > 1 }.keys.toSet()

DuplicatesSpec.kt

import io.kotest.core.spec.style.DescribeSpec
import io.kotest.matchers.shouldBe

class DuplicatesSpec : DescribeSpec({
    describe("duplicates") {
        it("returns empty set when no duplicates found") {
            val list = listOf(1, 2, 3, 4, 5)

            list.duplicates() shouldBe emptySet()
        }

        it("gets duplicates") {
            val list = listOf(1, 2, 2, 3, 4, 4, 4, 5)

            list.duplicates() shouldBe setOf(2, 4)
        }
    }

    describe("duplicatesBy") {
        it("returns empty set when no duplicates found") {
            val list = listOf("a" to 1, "b" to 2, "c" to 3)

            list.duplicatesBy { it.second } shouldBe emptySet()
        }

        it("gets duplicates for items grouped by a custom selector") {
            val list = listOf("a" to 1, "b" to 2, "c" to 2, "d" to 2, "e" to 3, "f" to 3, "g" to 4)

            list.duplicatesBy { it.second } shouldBe setOf(2, 3)
        }
    }
})