Microsoft is aware of this bug (you can find longstanding reports of it on their site in threads like this one), but there’s currently no known workaround. For example, the bug can happen if you send a message to someone using an iPhone, but will not happen if the recipient is using Outlook, or if a Microsoft Exchange mail server is anywhere in the mail path. The duplicated images can appear when you send messages to people who are not using Microsoft software to read the messages. Both Outlook 2011 for Mac and Outlook 2016 for Mac have a bug that can cause duplicated images in some cases.
