Validation for duplicated on form

array_unique is a php function. A custom rule like this will probably work:
See: https://codeigniter4.github.io/userguide...stom-rules

PHP Code:

<?php

class MyRules
{
    public function duplicates($serialarray): bool
    {
$originalLength = count($serialArray);
$duplicateArray = array_unique($serialArray);
$duplicateLength = count($duplicateArray); 
if ($originalLength <> $duplicateLength){
return 0;
}else{
return 1;
    }
}