duplicates 方法从集合中检索并返回重复的值：

$collection = collect(['a', 'b', 'a', 'c', 'b']);

$collection->duplicates();

// [2 => 'a', 4 => 'b']Copy

如果集合包含数组或对象，则可以传递希望检查重复值的属性的键：

$employees = collect([
    ['email' => 'abigail@example.com', 'position' => 'Developer'],
    ['email' => 'james@example.com', 'position' => 'Designer'],
    ['email' => 'victoria@example.com', 'position' => 'Developer'],
])

$employees->duplicates('position');

// [2 => 'Developer']Copy