Duplicate emails sent when using email apps other than Yahoo Mail

For the last couple of weeks, emails I send from my sbcglobal email account using the native app on my Samsung S8+ or Outlook for Android are sent twice, and two copies show up in my Sent folder, one marked read, and the other (which appears about 30-60 seconds later) marked as unread. However, if I send an email from the same account through Outlook 365 on my laptop, only a single copy is sent. I read on a different message board that this appears to be an issue with changes to Yahoo Mail security, because the duplicate problem does not occur with the Yahoo mail app on an Android phone. I don't want to have to use the Yahoo Mail app to avoid this problem. Please advise. Thank you.