a single email group distribution list causing duplicate email sends to members

H i there team. I hope you're day is going well.

I am trying to figure out why I'm getting duplicate sent while using our distribution lists.  This just started in the last couple of months.

I of ten send emails to our distribution lists:
guests-9051@toastmastersclubs.org
prospects-9051@toastmastersclubs.org
members-9051@toastmastersclubs.org

Lately when I send one email to all 3 different email address, 3 emails are received by the members.
Today I tested by:
1.  sending 3 emails:
     one email to each of the 3 individual email addresses.  No problem.
2.  sending one email to 2 groups email below.  No problem.
     guests-9051@toastmastersclubs.org
     prospects-9051@toastmastersclubs.org
3. However when I sent one email to all 3 emails addresses.... - Problem
     3 emails were received by all the members in the members-9051@toastmastersclubs.org group.

Please help me resolve this issue.  Its very frustrating.

Thank you!
Michele 
Cape Coral club #9051

You can only send to one list at a time.

that sucks.  but thanks for letting me know