Hi Fred,
No worries about the duplication, if the solution – if provided – doesn’t work for you. 🙂
Thanks for checking for plugin conflicts. Did you / can you also check with using a default theme like twentyfourteen or twentysixteen?
Do you possibly have any customization on your site? Some code in functions.php that could affect this?
I also see that you are using a quite old version of WordPress. I would strongly recommend to update it to the most recent version (after backing up), as an old version could already cause a lot of issues, not to speak about security.
Waiting for your answer.
Cheers,
Andras