Duplicate email after AT&T/Yahoo mail merge

There's some other few things I don't understand about that image you show. 

1. Just how are you getting previews like that of multiple messages if they truly are multiple messages.  Both have the same date and time so I don't think they are multiple messages.  Or is that what you see when you click on a message to view it?
2. Where does that icon in the toolbar to the left of the up and down triangles on the right come from, i.e., the one with the two short horizontal bars?  It almost looks like a "collapse"/"expand" function of some sort.  I don't have that in my att/yahoo webmail display.
3. Just below that it says "Yahoo/inbox" while mine says "Att/Yahoo... /Inbox" which only shows when I display a message (see 1 above).

I cannot find any settings combinations that produce that kind of display so I wonder if you are actually getting a duplicate email and it's just not displaying the message in the window twice.  While you logos in the upper left has the att logo to the left of "Yahoo", which is correct, I cannot explain the display differences.  The one that says "Yahoo/Inbox" is the most curious.  It's almost as if you are using yahoo webmail and not att/yahoo webmail but I cannot explain the logos if that is the case.  I don't have a yahoo email account so I cannot say whether these differences are in their display.

Now that they have merged the webmail clients with yahoo, i'm getting my emails duplicated every time.

Actually att and yahoo have (or continually attempting) to be unmerging the att accounts from yahoo.  And the att/yahoo webmail has it's own site accessed through start.att.net, att's own home page.

Here is how the emails come in.   Inbox shows 1 message, but under the actually main section of the inbox it shows 2.   And I actually have to delete both of them as if they are 2 individual e-mails.   More of a nuisance than anything really.

https://imgur.com/mRhLb62

I'll give it a shot and see if this fixes it. A lot of the time the duplicates tend to come in with junk/spam emails that make it to my inbox lol.

Thank you the suggestion you mention took care of the issue.

I have a separate problem.  All 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.  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 if one uses the new 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.

I have had the same problem for about two weeks or so.  I have 2 copies of emails in my Sent box for each email I send.  I have 2 email accounts, and it is happening with both of them regardless of whether I send from my phone or my computer.  I'm not using the Yahoo mail app.  To make matters worse, when I send a test message to myself, I also receive 2 copies of the email.  Any resolution yet? 

I'm sorry, I don't understand. What I posted is not a solution.  It is a variant of the same problem outlined above by others. To my knowledge the recipients are receiving just one email. However, 2 copies appear in my sent emails.  I am hoping someone can offer a suggestion to remedy this.

count me in -- same problem - only started recently: send a message from iPhone mail or outlook on imac and yahoo mail on the web, iPhone mail, outlook imap on my imac, etc all show two copies of the same sent email in the sent folder (one with blue dot and one without blue dot). turning off "save copy" (to server) in imap account advanced settings on outlook doesn't change anything.