Workflow for managing duplicate cases
There is a merge case feature. The clients need to be the same client on both cases so you may need to merge contacts or change the client first. The feature is a little blue link about 1/4 way down the manage case screen.
You may also want to look at https://lab.civicrm.org/extensions/mergecaseandkeepid since, for unfortunate historical reasons, the case id changes when you do a merge.