Cases can become duplicated in many ways, like when a customer sends 2 emails at the same time. Now you have two cases with relevant information for the same problem.
CloudAnswers Case Merge allows you to merge the two cases, keeping email history from both cases on one case using a process very similar to the standard lead and contact merge functions in salesforce.
Here you can see it in action, a user has 2 cases from the same customer related to the same topic. They select the two cases, click "Merge Cases", confirm the record to keep, and then get a single case with all email history combined: