Ok, after a bit of research and digging in the dev tools, I found the double submission was caused by the WordPress plugin AntiSpam by CleanTalk.
This is a plugin that monitors every form field on a page, preventing the spam.
Disabling it solved the issue.
Anyway, I wanted the plugin enabled, so I simply set an exclusion in the advanced options.
I used mauticform as string to exclude and then checked the “Use regular expression in field exclusions”: now all fields that contain mauticform are ignored by Antispam by CleanTalk and the double submission problem is solved.
Thank you @ricardomartins to have put me on the right path!