While CASEpeer can merge contacts, cases can not be merged. Here are the steps to combine duplicate cases:
Identify which case is most complete - update the extraneous case status to Error/Duplicate.
Open both cases side by side, each in their own browser tab, and add in case information to the case you are keeping within CASEpeer.
If necessary, download any documents and re-upload them to the correct case.
Once all necessary case information is added, delete the duplicate case.