The error message you're seeing probably looks like this:

This is usually caused by a duplicate Outlook Data File.  As illustrated in the example screenshot below, there are two references to an Outlook.pst.  See the Comment column.  One Data File is the Default Data File.  Removing the duplicate (non-Default) Data File should resolve this issue.  Contact your local IT personnel if you require clarification about what can and can't be deleted.

Click to enlarge