Multiple emails when using distribution lists

[mpapier]

In Exchange 2003 we had smaller distribution lists (for example "dept a" and "dept b") that were in turn members of other lists (for example "everyoneDL").

If a user or contact was a member of both "dept a" and "dept b", they still only received one copy of an email sent to "everyoneDL."

Now that we have completed our migration to Exchange 2007, these users who are members of both "dept a" and dept b" receive two copies of emails sent to "everyoneDL." Any way to control this behavior so only one email is delivered?

Thank you very much!