How to remove duplicate emails sent via browser in Outlook?

Question

Yandex

Broke my brain.
There is Outlook (2013-2016 like) where letters from three mailboxes (via POP3) are collected in the public inbox.
There is a corporate mail on Yandex. When sending a letter from the web interface, two copies of the sent letter fall into the inbox.
There are no filters and redirects in the mail.
Most likely everything is very simple, nevertheless, for three days I can not find the cause and fix it