Ok heres a basic rundown of what you need to do .

1 Make sure that the object you want to duplicate has it's center verts on the center Axis Line.

You can select one or all the edge verts and switch to the move tool ( in the options of the move tool turn off Keep Component Spacing)the slect the X- axis only on the move tool .Now hold down Ctrl X and Middle button drag over the Center axis on the grid and the verts will snap to the center.

2 Now you have to make sure your pivot is also on the Center axis of the grid.

To do this Select the geometry you want to mirror over the switch to the move tool.Hit insert (Ins) on the keyboard and the hold down ctrl + X and once again Middle button drag over the Center axis on the grid.
Finally hit insert again to complete the pivot move.

3 Duplicate your object then in the channel box (top right) type in -1 in the scale X.

4 Select both pieces of geometry and go to Mesh and then hit Combine.

5 Select the vertices in the center that need joining and under Edit Mesh pick the option box of the Merge and set the Threshold to 0.001 then hit apply.

You should now have one object completely joined.