From the first posted screenshot, it might be that you didn't merge the vertices that were on top of each other.

After you have duplicated and merged the two half poly bodies together, go to the Front view.

Select the vertices that lie in the middle. (Careful not to select those that are not in the middle).

Then choose to Merge Vertices. (You might need to change some numbers in the option box)

Do update us when you find the solution.