Getting duplicate messages saved to 'Sent' folder

Getting duplicate messages saved to 'Sent' folder

When I send a message from Android phone (K-9) imap client or when I send a message from Windows using Thunderbird, two identical copies of my outbound message are saved to Sent folder.

When I send a message from zoho web mail page, I believe there is no duplicate of my outbound messages.

Have I misconfigured something?  What do I need to do to fix it?

thanks!
Linda