Sometimes there are parts/assys which can be used like a symmetric item concerning mounting and function, although they are not really topologically symmetric (e.g. pneumatic cylinders, electric drives etc.).
If one could define a user-defined symmetry as a special feature or a special coordinate system added to this item, the built-in mirror function should be able to handle this feature and to re-position it in the (mechanically) correct way instead of creating a mirrored new one.
An optional warning may be applied if this happens like when you load a package with containers