comment replies duplicates original comment

That sounds really strange. I suspect a connection with a plugin you are using or the theme you are using. Could you test deactivating plugins to exclude them? The troubleshoot mode of Health Check could be helpful: https://wordpress.org/plugins/health-check/

Please use the troubleshoot mode mentioned above. Is described in more detail here: https://make.wordpress.org/support/handbook/appendix/troubleshooting-using-the-health-check/#troubleshooting

I would recommend that you contact your theme’s support for this.