wp_fun95 on "[Plugin: Contact Form DB] Preventing duplicate e-mail addresses in database"

Hi Michael
did you have some time to think about this problem?
in the meantime I asked again support from the plugin who told me that editing the core files is not recommended (so they do not exclude this) and that any change would be lost in every update of the plugin.
Any ideas of which are the core files referred to (definitely not the functions.php, since any change in it shuts down the theme and the whole site)?
many thanks in advance for your help
regards