First your duplicate special looks to have the correct settings. You might want to try deleting history on the part and try again when you get odd results like that. Also, when in doubt reset the tool and then make your settings.
But unless you wanted to make an instance copy of the object using duplicate special is really kind of overkill.
What would have been simpler is to mirror and weld the seam as Dave suggests or I like to use the marking menus and do this...
1. select the object
2. shift-rmb
3. select mirror->mirror +x (in your case)
Or you could simply...
1. press ctrl-d to duplicate the object
2. set the scale of the dup to -1 in the channel box
3. normals>reverse
4. mesh>combine
5. then select and merge the verts
Which if you get used to using the marking menus is faster then messing around with duplicate special. options I tend to use duplicate special when I want to make an instance copy, or I want to duplicate the input nodes, or I want to make multiple copies and not for simple duplicate and scale -1 operations.
"If I have seen further it is by standing on the shoulders of giants." Sir Isaac Newton, 1675