I'm having the following issue: when duplicating a (Custom) Post, the Relationship I set up does not copy with the post.
For duplication, I use the "Duplicate Page" plugin, which normally copies all the data of a post.
The relationship data is stored in a custom table, not like the post meta which is stored in the default table of WordPress (wp_postmeta) so it wouldn't be duplicated. You can tell the plugin author to follow our documentation to create an integration with Meta Box. https://docs.metabox.io/extensions/mb-relationships/