TYPO3 Core

For TYPO3 11 (and 12) records with a bidirectional MM relations to themselves have duplicate MM records after copying them via DataHandler.

A prominent example would be EXT:news with its two columns related and related_from (stripped down to relevant parts):

EXT:news/Configuration/TCA/tx_news_domain_model_news.php:

        'related' => [
            'config' => [
                'type' => 'group',
                'allowed' => 'tx_news_domain_model_news',
                'foreign_table' => 'tx_news_domain_model_news',
                'MM_opposite_field' => 'related_from',
                'MM' => 'tx_news_domain_model_news_related_mm',
            ],
        ],
        'related_from' => [
            'config' => [
                'type' => 'group',
                'foreign_table' => 'tx_news_domain_model_news',
                'allowed' => 'tx_news_domain_model_news',
                'MM' => 'tx_news_domain_model_news_related_mm',
                'readOnly' => 1,
            ],
        ],

• Install EXT:news
• Create a folder with 2 news records, one refering to the other (via related column)
• Copy that folder

Original folder	Copied folder

Expected result: 2nd folder with 2 news records, one refering one time to the other
Actual result: 2nd folder with 2 news records one refering three times to the other

With TYPO3 10 this worked as expected. And since the TCA in news hadn't changed, I assume it's a TYPO3 core issue. Plus I can reproduce it with pages records as well.

Hint: when unsetting the TCA for the related_from field, it's working flawlessly.