Issue with customer Grid collection when used UNION in Magento2

I am rendering customer grid in the admin panel with some additional fields from the custom table.

The grid is throwing with duplicate column error.

I have used the below code.

Vendor/Module/etc/adminhtml/di.xml

<?xml version="1.0" encoding="UTF-8"?>
<config xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
    xsi:noNamespaceSchemaLocation="urn:magento:framework:ObjectManager/etc/config.xsd">

<type name="MagentoCustomerModelResourceModelGridCollection">
    <plugin name="sales-rep-customers"
            type="VendorModulePluginCustomerModelResourceModelGridCollection"
            sortOrder="90"/>
</type>

Vendor/Module/PluginCustomerModelResourceModelGridCollection.php

<?php

namespace VendorModulePluginCustomerModelResourceModelGrid;
use MagentoCustomerModelResourceModelGridCollection as Subject;
use MagentoBackendModelAuthSession;
use MagentoFrameworkAppResourceConnection;

class Collection
{
/**
 * @var Session
 */
private $adminSession;
public function __construct(
    Session $adminSession,
    ResourceConnection $resourceConnection
)
{
    $this->adminSession = $adminSession;
    $this->connection = $resourceConnection->getConnection();
}
public function aroundLoadWithFilter(
    Subject $subject,
    callable $proceed,
    $printQuery = false,
    $logQuery = false
)
{
    if (!$this->adminSession->isLoggedIn()) {
        return $proceed($printQuery, $logQuery);
    }

 $select =  $this->connection->select();
    $selectQuery1 = ''; // returns select statement with some conditions
    $selectQuery2 = ''; // returns select statement with some conditions
    $newest = $select->union([$selectQuery1, $selectQuery2]);
    $subject->getSelect()->reset();

    $subject->getSelect()->from(
        [
            'main_table' => $newest
        ]
    );

 return $proceed($printQuery, $logQuery);

So, the select statement that is returned in the variable $newest if I execute the query in mysql/phpmyadmin , I am getting the result, but in the Grid the error is throwing when customer grid opened in the admin.

Please someone look into this and provide some inputs. Thanks in advance!