Lately I've been seeing duplicate lists on my record detail pages. For example I see two Cases lists on a Contact's detail page, or duplicate Opportunity lists. Did someone else encounter that?
Have you got multiple lookup fields from Case to contact? For example, if you have a "Reporting Contact" and an "Escalation Contact" for a Case then this can lead to two related lists on the Case (one for each relationship).
Go to setup | Customize | Cases and see if there is more than one field defined Lookup(Contact).
You can also go into the field definition and change the "related list label" so that you can distinguish between the two.