Samsung email app w/Yahoo, my email responses duplicate within email, doesn't show on phone

[serenifly]

Yes, I know there is a Yahoo mail app, I hate it and find it really hard to use. Yes I am a technological *****, just trying to keep afloat here.

So when I respond to emails using my phone, it does not look like it is duplicating when I send and ALSO does not show the duplication from the Sent folder from the email app. However, when others rec my email and when I look them up in the Sent folder on a computer later, the duplication is there.
Any idea why it doesn't show the duplication on the phone app, but it is truly duplicating and how to fix? Hasn't always done it, deleting and reattaching email addresses did not help.