When you get error like: Item (MagentoCatalogModelProductInterceptor) with the same ID “<id>” already exists.

Then you can use below module to resolve above error.

1. Create new folders Magemonkey/DuplicateEntry in app/code magento folder.
2. add new file registration.php in DuplicateEntry folder and put below code.

<?php

MagentoFrameworkComponentComponentRegistrar::register(
 MagentoFrameworkComponentComponentRegistrar::MODULE,
 'Magemonkey_DuplicateEntry',
 __DIR__
);

3. add new file etc/module.xml in DuplicateEntry folder and put below code.

<?xml version="1.0"?>

<config xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:noNamespaceSchemaLocation="urn:magento:framework:Module/etc/module.xsd">
 <module name="Magemonkey_DuplicateEntry" setup_version="1.0.1"></module>
</config>

4. add new file etc/di.xml in DuplicateEntry folder and put below code.

<?xml version="1.0"?>

<config xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:noNamespaceSchemaLocation="urn:magento:framework:ObjectManager/etc/config.xsd">
 <type name="MagentoEavModelEntityCollectionAbstractCollection">
    <plugin name="find_duplicate_entry" type="MagemonkeyDuplicateEntrypluginCollection" sortOrder="20"/>
 </type>
</config>

5. add new file plugin/Collection.php in DuplicateEntry folder and put below code.

<?php
/**
 * Copyright © Magento, Inc. All rights reserved.
 */
namespace MagemonkeyDuplicateEntryplugin;
use MagentoFrameworkDataCollectionEntityFactoryInterface;
use MagentoFrameworkOptionArrayInterface;

class Collection
{
    /**
     * @param MagentoEavModelEntityCollectionAbstractCollection $subject
     * @param Closure $process
     * @param MagentoFrameworkDataObject $dataObject
     * @return $this
     */
    public function aroundAddItem(MagentoEavModelEntityCollectionAbstractCollection $subject, Closure $process, MagentoFrameworkDataObject $dataObject)
    {
        try{
            return $process($dataObject);
        }catch ( Exception $e){
            return $this;
        }
    }
}