From 7fe20ad1b38ed3077c3666bca86b0c89f820c9f8 Mon Sep 17 00:00:00 2001
From: "b.van.rijswijk" <b.van.rijswijk@socialblue.com>
Date: Tue, 7 Apr 2020 17:48:21 +0200
Subject: [PATCH] Adds duplicates function

---
 src/Collection.php            | 12 ++++++++++++
 tests/unit/CollectionTest.php |  7 +++++++
 2 files changed, 19 insertions(+)

diff --git a/src/Collection.php b/src/Collection.php
index 986618f..5078951 100644
--- a/src/Collection.php
+++ b/src/Collection.php
@@ -178,6 +178,18 @@ public function unique()
         return new static(array_unique($this->items, SORT_REGULAR));
     }
 
+    /**
+     * Returns a collection with duplicate values
+     * @return Collection
+     */
+    public function duplicates()
+    {
+        $unique = array_unique($this->items);
+        $duplicates = array_diff_assoc($this->items, $unique);
+
+        return new static($duplicates);
+    }
+
     /**
      * Return a new collection with the items reverese
      * @return Collection
diff --git a/tests/unit/CollectionTest.php b/tests/unit/CollectionTest.php
index c40dc86..1411370 100644
--- a/tests/unit/CollectionTest.php
+++ b/tests/unit/CollectionTest.php
@@ -195,6 +195,13 @@ public function test_unique_removes_duplicate_values()
         $this->assertEquals([1, 2, 3], $unique->values()->all());
     }
 
+    public function test_duplicate_values_in_collection()
+    {
+        $this->assertEquals(['Bram'], (new Collection(['Bram', 'Bram', 'John', 'Doe']))->duplicates()->values()->all());
+        $this->assertEquals([1, 45], (new Collection([1, 1, 2, 5, 11, 28, 45, 45, 60]))->duplicates()->values()->all());
+        $this->assertEquals([], (new Collection(['Bram', 'John', 'Doe', 'Jane']))->duplicates()->values()->all());
+    }
+
     public function test_reverse_items_in_collection()
     {
         $collection = new Collection([1, 2, 3]);