duplicate email from Yahoo mail
For the last several weeks I get duplicates of every email I sent to my businesses partner. It is very clearly a yahoo problem. If it's not fixed within a week I'm forwarding all my emails to my gmail and deleting this account. It's completely obnoxious, and I tried for hours to reach anyone at Yahoo with a complaint and every one of their links is broken from their own help section.
Please check the third-party email apps you are using to access Yahoo Mail. Make sure the settings are IMAP not POP: https://help.yahoo.com/kb/SLN4075.html
Try deleting and readding the account to the tird-party app.