eNewsletter- user sign up for multiple groups?

I have created mutiple groups. When i user signs up for a newsletter, they get signed up to the newsletter group they choosed and then they recieve a welcome mail.

That all worrks fine. But when the same user signs up for another group/newsletter, it does not get registrat. I can go in and manually assign more groups, but it beats the whole automated part..
Please advice...
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