It seems Max is still notorius for double edges. Take a plane and cut it in half. Now grab the two edges at one end and extrude them together. Insead of keeping one edge between the new geometry, it in fact adds an extra edge. Oddly enough you can weld the new verts together but the double edge remains. I am finding this issue godamn impossible to fix. Unless you count cutting out your polys and re-trying using a slow approact to modeling.
I am wondering if there is some kind of trick to extruding two connected edges together without max inserting an overlapping edge? It is also annoying because the two new edges are right ontop of each other and it often looks as though everything is ok. Until you export of course or use the meta-nurms trick for spotting it.