Add duplicate() function to Query Builder

Signed-off-by: davidarendsen <davidarendsen@hey.com>

tests/Functions/DuplicateFunctionTest.php

@@ -0,0 +1,20 @@
+<?php
+
+declare(strict_types=1);
+
+namespace Tests\Functions;
+
+use Arendsen\FluxQueryBuilder\Functions\Duplicate;
+use PHPUnit\Framework\TestCase;
+
+final class DuplicateFunctionTest extends TestCase
+{
+    public function testSimpleDuplicate()
+    {
+        $expression = new Duplicate('tag', 'tag_dup');
+
+        $query = '|> duplicate(column: "tag", as: "tag_dup") ';
+
+        $this->assertEquals($query, $expression->__toString());
+    }
+}