Contact list (category): other related emails being added to the list, though they are not part of the original list

Contact list (category): other related emails being added to the list, though they are not part of the original list

I've created a list of contacts (a Category), and when I use it to write an email, aside from the emails I've added to the list, other related emails are added to the email's recipients' list.
E.g: in the contact list (category) "Example List" there's John Doe < john.doe@example.com>; but when I start composing a new email and I add "Example List", both John Doe < john.doe@example.com> and John Doe < john.doe@otheremail.com> are added (of course, both are emails I have among my contacts, but they haven't been added to the list (category) in question, nor they are being shown as part of it when seen in the contact manager.