without seeing a pic, i would assume that you can reduce the number of triangles before performing the duplication. while keeping the side the same, merge edges, verts, etc. until you can reduce the number of triangles you have on the edge. if you can't reduce them, then the only other option i can think of is to merge all the poly portions into one object (polygons -->combine) and then merging edges/verts from there on.
at the same time, if you can do that just for one duplication, i'd recommend you do that and then just duplicate the new object half the time around.
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