another quick hint, if you are doing this way, you should have all the vertices you want to connect at 0-X so when you are modeling, and if you might use some kind of average vertices, or sculp geometry tool that the vertices might move around, just go 1 by 1 and snap them to the grid at the center, then when you duplicate it (and as ctbram said, make sure the pivot is also at the 0 coordinate) all the vertices will be in the same spot and if so you can actually drag select all of them (yes ALL of them) and merge them, (Maya will notice they are all grouped by 2, and merge only the overlapping vertices) it will save you a lot of time selecting each one of them snapping to the other and then merging it