When you export e-mail messages, you create a copy of the messages in the Exported file. Meanwhile, the original messages remain in the Mailbox. Result: two identical files.
You may be familiar with the process of exporting data. Be warned: In Outlook, exporting isn't like archiving. When you export messages, they stay in their original location and they're duplicated in a new file. This means that exporting messages doubles their number and the space they require.
Export when you want to copy messages—for example, if you want to use them in another program such as Microsoft Excel. Archive when you want to move messages from one place to another in Outlook.