Dagon Form Mailer v5.8 sends 2 notification emails

I’ve been a long-time user of Dagon Design Form Mailer. Currently have version 5.8.

Recently (don’t know when) I started getting two notification e-mails each time someone submitted to the form.

I reset defaults and reconstructed form, but I still get two notification e-mails.

Did this happen to other people? Where should I look if it’s not the plugin itself?

Thanks!