Hi @Matthew Beechey - Mobius ,
Welcome to our forum!
In order to further confirm the problem, please provide information about the following problems:
- What's the version of your Outlook? (File > Office Account > About Outlook). Please make sure you have upgraded your Outlook to the latest version.
- Do you use manual archiving or automatic archiving?
- You mentioned that duplicate messages will appear after moving deleted items, right? If so, how did you view it, by searching the mailbox and finding two identical emails?
According to your description, I noticed that you seem to have many concerns about the large mailbox size. As far as I know, exporting mail to backup is also a good way to reduce the mail size. For more information, please refer to: Export or backup email, contacts, and calendar to an Outlook .pst file.
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