In Outlook under Windows or Mac. If you begin to experience redundant emails from the same sender, or you are receiving multiple emails from the same sender, then likely you need to reset Outlook "Leave Messages on server" option.

Here are some steps to help you resolve this problem:

1. Try to restart your computer, check if the problem continues.

2. If the problem continues, then disable "Leave Message on server" by unchecking the box as following:

- Open your Mail account settings (This can be under Files -> Account Settings in Outlook 2013+,  or Tools -> Account Settings in prior versions)
- Highlight your email address
- Click on "Change"
- Click on "More Settings"
- Select "Advanced" tab
- uncheck the box "Leave copy of messages on server"

3. Restart your Outlook program

4. Repeat the same steps above and put back "Leave copy of messages on the server" if you wish to keep part of your messages on the server so you can see the same messages on your phone and computer.