Mirror Tool
The Mirror tool is used to duplicate a feature/body along a plane of symmetry.
To perform this function, select the feature/body that needs to be duplicated:
Click on the Mirror icon:
Finally, select the Mirroring Element (plane of symmetry) that you wish to mirror it about and click OK:
It is also possible to mirror just a single feature or group of features of a PartBody rather than the whole thing. To do this, use the “Define In Work Object” option (explained here) to exclude the part that you do not want to mirror.
This has been done below by mirroring the PartBody whilst excluding the hole.