From @daniel :
Looking at https://gerrit.wikimedia.org/r/plugins/gitiles/mediawiki/extensions/Duplicator/+/master/Duplicator.page.php it seems like Duplicator is incompatible with the MCR database schema (and will break for Actor migration and Comment migration as well).
To be MCR compatible, it will (depending on the $wgMultiContentRevisionSchemaMigrationStage) also copy the relevant rows in the slots table (entries in the content table can be shared between page copies), and possibly ignore the rev_text_id field. Similarly, depending on the value of $wgCommentTableSchemaMigrationStage and $wgActorTableSchemaMigrationStage, it needs to handle rev_user_text and rev_comment differently.
To remove the need for the extension to bind to the concrete database schema (not to migration stage config), it should probably create a new MutableRevisionRecord for each RevisionStoreRecord of the existing page, and use a PageUpdater to save them to the new page. This would completely hide any DB schema details, but it would also be slower. Some tweaks in core may be needed to enable sharing of content between pages, core currently only supports this between revisions on the same page at the moment.