Multiple copies of emails received by some members on member list

From Club #8700
When I send emails to all members from my Thunderbird email application using the members-8700@toastmastersclubs.org address, some members report receiving 3 copies of the same email each time and a couple of them are mildly annoyed. Any idea why this might be happening? Each member is only on the list once under a single email address. Could this be due to settings in their own email programs or do I need to stop sending from Thunderbird? Thank you in advance for anything you can offer.
VP Membership, Club 8700

Did you have more that one list on your To or CC line?
You can one send to one (1) list per email.
From what I can see you send to member and guests in the same email.