snipe-it/app/Http/Traits/UniqueSerialTrait.php

<?php
namespace App\Http\Traits;

trait UniqueSerialTrait
{

    /**
     * Prepare a unique_ids rule, adding a model identifier if required.
     *
     * @param  array  $parameters
     * @param  string $field
     * @return string
     */
    protected function prepareUniqueSerialRule($parameters, $field)
    {
        if ($settings = \App\Models\Setting::first()) {
            if ($settings->unique_serial=='1') {
                return 'unique_undeleted:'.$this->table.','. $this->getKey();
            }
        }
    }
}