When working with TYPO3 and inline fields in the TCA configuration, TYPO3 already handles the duplication of related child records when you copy a parent record, as long as the configuration is properly set up.

This behavior is managed in the DataHandler class, specifically in the copyRecord() method, which ensures that referenced child records are duplicated. You can refer to the source code for more details : https://github.com/TYPO3/typo3/blob/12.4/typo3/sysext/core/Classes/DataHandling/DataHandler.php#L4117

In your case, you should verify that the 'tx_headline' field is correctly storing the UID of the parent record in the database.