Mirroring and Duplicating Special with an offset
these seem to be two ways to create a copy of your geometry over an axis. With special duplication, any changes made will be copied over the axis. I get that much.
But, when I try and mirror my geometry, I get problems. If I do it without merging the halves, it is fine - but I need to then merge all the central vertices. When I set the parameter to merge the vertices, my mesh is badly deformed (not at home so no images just now). Say it is a head, with the mouth ajar - the mouth is filled and pinched and badly deformed (all my center vertices are at 0, so that's not the problem). It removes some of my vertices and adds new ones in the center.
Is there a way around this? Or, is using a duplicate special an acceptable alternative (not sure yet is they are for finished products or more for previewing).
Sorry i've no images just now but I think someone will know what I mean.