how to avoid duplication of contacts when added to a group

In the apple address book, i would like to create a distribution lists for sending an email to a group of people. I therefore create a group. Then drag and drop the contacts from my 'all contacts' to this group.

To my surprise i see that the address book creates a duplicate of every contact added to a group. And the info of that contact is not synchronized between the group and the 'all contacts'.

I tried deleting the duplicate contact; but it then either removes it from the group or from the 'all contacts'.

This can really not be the purpose of managing contacts and distribution lists in the apple address book.

or ???