Blender addon
Duplicate and mirror a shape key with a single click (Technically 2). It also supports left/right naming scheme if detected (ex: browDownLeft -> browDownRight) otherwise just adds _mirrored to the name.
Option can be found in the "Shape Key Specials" dropdown in the Shape Keys tab. Select the shape key you wish to duplicate and mirror, click on "Duplicate and Mirror Shape Key" or Duplicate and Mirror Shape Key (Topology)". The generated Shape key should appear under the selected one.
Select "Edit -> Preferences" and choose "Add-ons" tab. Click "Install from disk..." (little arrow dropdown at top right) and choose the downloaded file. And thats it,