Add assertk List extension containsNoDuplicates to :core:testing

core/testing/build.gradle.kts

@@ -1,3 +1,7 @@
 plugins {
     id(ThunderbirdPlugins.Library.jvm)
 }
+
+dependencies {
+    implementation(libs.assertk)
+}

core/testing/src/main/kotlin/assertk/assertions/ListExtensions.kt

@@ -0,0 +1,13 @@
+package assertk.assertions
+
+import assertk.Assert
+import assertk.assertions.support.expected
+import assertk.assertions.support.show
+
+fun <T> Assert<List<T>>.containsNoDuplicates() = given { actual ->
+    val seen: MutableSet<T> = mutableSetOf()
+    val duplicates = actual.filter { !seen.add(it) }
+    if (duplicates.isNotEmpty()) {
+        expected("to contain no duplicates but found: ${show(duplicates)}")
+    }
+}

core/testing/src/test/kotlin/assertk/assertions/ListExtensionsKtTest.kt

@@ -0,0 +1,28 @@
+package assertk.assertions
+
+import assertk.assertThat
+import kotlin.test.Test
+
+class ListExtensionsKtTest {
+
+    @Test
+    fun `containsNoDuplicates() should succeed with no duplicates`() {
+        val list = listOf("a", "b", "c")
+
+        assertThat(list).containsNoDuplicates()
+    }
+
+    @Test
+    fun `containsNoDuplicates() should fail with duplicates`() {
+        val list = listOf("a", "b", "c", "a", "a")
+
+        assertThat {
+            assertThat(list).containsNoDuplicates()
+        }.isFailure()
+            .hasMessage(
+                """
+                expected to contain no duplicates but found: <["a", "a"]>
+                """.trimIndent(),
+            )
+    }
+}