It's really stupid in the 3.0 series as it was really simple(still is, but not overly simple like it was.
1. Select your geometry
2. Go to Edit > Duplicate/Instantiate > Clone Single*
3. It will automatically selected so all you have to do is scale it -1 in whatever axis you want; Usually "X" axis; it is center dependant, but most stuff should be zeroed to the global axis.
* In earlier versions you could use an instance by pressing shift-I, however, in 3.x series your geometry needs to be under a model node to do this. Clone works just the same, but no standard hot key.
That's what clone or instance does; it moved with the original; you just need to scale in -1.
All you have to do is one youy close go type "-1" in the x input box to the right under "S" for scale and you have a symetrical clone; just don't freeze it, only free the original.
However, I am guess you used dscowboys way because duplicated don't update.