If it isn't welded to the main portion of the model, just go into element mode, and select that element, and delete. If it is welded, try to delete only the portion of the model that is welded, until you can select the second copy as an element.
assuming you use max, select "elements" and delete the proper ones ? or the cut and merge is not result of a internal copy, but the consequence of max's illogic behavior (see the Lightwave > 3dsmax 9 thread)