Problem with nested groups

Is there a way to prevent emails that sent to a group containing nested groups from being delivered 2 times if someone is in more then 1 group?

eg:
Main group contains SubGroup1 & SubGroup2

UserA is in SubGroup1 & SubGroup2 and gets the email twice when a message is sent to the Main group.

I've just conducted a little test, but the result tells me what I allready know that should happen:

Set up a group containing:

• grp1: 2 colleagues and me
• grp2: same 2 colleagues

Test1:

• main group: Mail only
• grp1: multipurpose
• grp2: multipurpose
• Result: 1 mail

Test2:

• main group: multipurpose
• grp1: multipurpose
• grp2: multipurpose
• Result: 1 mail

This is on Notes 6.5.4 and Domino 7.0.2FP2 on Windows, and all real groups are multipurpose.
Can anyone shine a light on this?