Install the Quickmail plugin https://github.com/lsuits/quickmail
Turn editing on.
Add the Quickmail block to your course page.
Add the Activities block to your course page.
Click the Resources link in the Activities block.
Add the Quickmail block to the Resources page.
Then try to backup or import the course.
You will receive an error "error/setting_already_added".

Other pages like the forum do not allow you to add the Quickmail block to them and do not have this issue.

I could not recreate this with blocks other than Quickmail. I think that is because Quickmail has it's own logs and config data involved in the backup process versus most blocks which simply display information from other places within Moodle.