[Mailman-Users] Problem with duplicate emails

[Javad Hoseini-Nopendar]

Hello
I created two mailing lists on my website:
newsletter at iranravi.com
And:
iranravi at iranravi.com

Also, IranRAVI at IranRAVI.com is a member of newsletter at IranRAVI.com. That 
means, when I send an email to Newsletter at iranravi.com, if someone is not 
the member of this group, but is the member of IranRAVI at IranRAVI.com, this 
person will also receive the email. In fact, one group is only for sending 
newsletters and the other group is for discussing about various matters. now 
I have a problem. When I send an email, if someone is the member of both 
groups, this person will receive two copies of the email. Is there any way I 
can prevent duplicate emails? I saw a choice in General options:
Prevent Duplicate Emails.

I thought mailman would delete the duplicates automatically, but it seems 
not to be the case. Please help.

Thank you