Hi Clay,
Can try once xpath is like “not(.=preceding::d:OE/@NID)”.
Before trying for above xpath you need to test first of all right side field xpath is correct or not. For this, you can try “d:OE/@NID” xpath is default value for one field and that field which is same level “d:OE” only. If you gets value then our above xpath is correct. By this way you can figure it out where is the problem of your xpath.
Hope this Help you