A common issue for users of Microsoft mail clients (Outlook, Outlook Express) is multiple copies of messages showing up in your Inbox with no obvious reason.
The most common cause of duplicate messages may be the option to Leave Messages on Server. If you set the Leave Messages on Server setting to more than 5 days, you will almost certainly experience terrible bouts of duplicate messages sooner or later!
Adjust this setting to 5 or fewer days like this in Outlook:
Tools → E-Mail Accounts → View or Change… → Change → More Settings → Advanced
If Leave Messages on Server is checked, make sure the Remove From Server setting is 5 or less.
Rules Processing
Another common cause of multiple messages are misconfigured E-Mail Rules. Check for this in Outlook by clicking:
Tools → Rules and Alerts
Check each rule to verify the proper logic.
Improper forwarding can also result in duplicate messages. Make sure you have not configured an account or a mail client to forward messages in an illogical fashion.