Click the Keep media icon once you have decided which media to keep (the Merge Approval checkbox must be selected on the other media asset)

This message box will appear, asking you to confirm that you wish to merge the duplicates of this group. Click Yes to continue.

If you have not selected the Merge Approval checkbox on another asset, you will see this message:

If media is merged, it will be logged in the History tab to show what has happened.