duplicate mesh
mirror mesh
combine meshes
merge verticies


you can just hit ctrl+d to duplicate and then type -1 in the scale each time you want to duplicate things
but if you've got a lot to mirror on the same axis (but not all at once) you just use duplicate special once and after that hit Ctrl+shift+d

setting up duplicate special takes as much time as doing it manually step by step but you get your rewards after the second time of duplicating another mesh the same way