ContainDuplicates should print them (#20) (#3846)

`containDuplicates` should list duplicates that violate the assumption,
for instance:
```
           shouldThrowAny {
               listOf(1, 2, 3, 4, 2, 1) shouldNot containDuplicates()
            }.shouldHaveMessage("Collection should not contain duplicates, but has some: [1, 2]")
```

kotest-assertions/kotest-assertions-core/src/commonMain/kotlin/io/kotest/matchers/collections/duplicates.kt

@@ -1,5 +1,6 @@
 package io.kotest.matchers.collections
 
+import io.kotest.assertions.print.print
 import io.kotest.matchers.Matcher
 import io.kotest.matchers.MatcherResult
 import io.kotest.matchers.should
@@ -35,10 +36,18 @@ fun <T> Collection<T>.shouldNotContainDuplicates(): Collection<T> {
 }
 
 fun <T> containDuplicates() = object : Matcher<Collection<T>> {
-   override fun test(value: Collection<T>) = MatcherResult(
-      value.toSet().size < value.size,
-      { "Collection should contain duplicates" },
-      {
-         "Collection should not contain duplicates"
-      })
+   override fun test(value: Collection<T>): MatcherResult {
+      val duplicates = value.duplicates()
+      return MatcherResult(
+         duplicates.isNotEmpty(),
+         { "Collection should contain duplicates" },
+         {
+            "Collection should not contain duplicates, but has some: ${duplicates.print().value}"
+         })
+   }
 }
+
+internal fun<T> Collection<T>.duplicates(): Collection<T> = this.groupingBy { it }
+   .eachCount().entries
+   .filter { it.value > 1 }
+   .map { it.key }

kotest-assertions/kotest-assertions-core/src/jvmTest/kotlin/com/sksamuel/kotest/matchers/collections/CollectionMatchersTest.kt

@@ -3,6 +3,7 @@ package com.sksamuel.kotest.matchers.collections
 import io.kotest.assertions.shouldFail
 import io.kotest.assertions.throwables.shouldNotThrow
 import io.kotest.assertions.throwables.shouldThrow
+import io.kotest.assertions.throwables.shouldThrowAny
 import io.kotest.assertions.withClue
 import io.kotest.core.spec.style.WordSpec
 import io.kotest.equals.Equality
@@ -230,6 +231,12 @@ class CollectionMatchersTest : WordSpec() {
             listOf(1, 2, 3, 3).shouldContainDuplicates()
             listOf(1, 2, 3, 4).shouldNotContainDuplicates()
          }
+
+         "print duplicates in message" {
+            shouldThrowAny {
+               listOf(1, 2, 3, 4, 2, 1) shouldNot containDuplicates()
+            }.shouldHaveMessage("Collection should not contain duplicates, but has some: [1, 2]")
+         }
       }
 
       "singleElement" should {

kotest-assertions/kotest-assertions-core/src/jvmTest/kotlin/com/sksamuel/kotest/matchers/collections/DuplicatesTest.kt

@@ -0,0 +1,22 @@
+package com.sksamuel.kotest.matchers.collections
+
+import io.kotest.core.spec.style.StringSpec
+import io.kotest.matchers.collections.duplicates
+import io.kotest.matchers.collections.shouldBeEmpty
+import io.kotest.matchers.collections.shouldContainExactlyInAnyOrder
+
+class DuplicatesTest : StringSpec(){
+   init {
+       "return empty list" {
+          listOf(1, 2, 3, 4, null).duplicates().shouldBeEmpty()
+       }
+
+      "return not null duplicates" {
+         listOf(1, 2, 3, 4, 3, 2).duplicates() shouldContainExactlyInAnyOrder listOf(2, 3)
+      }
+
+      "return null duplicates" {
+         listOf(1, 2, 3, null, 4, 3, null, 2).duplicates() shouldContainExactlyInAnyOrder listOf(2, 3, null)
+      }
+   }
+}