The default option for the mirror command in Revit is to create a copy. This is extremely annoying to most of us veteran cad users. The work around for this is the Ctrl key.
1. Select the object you want to mirror, then select the mirror command.
2. Now Revit asks you to select the axis of reflection. Before selecting your axis hold down the Ctrl key to deselect the copy option.
This method also works with the rotate and offset commands.
