[Koha] MARC import in Koha GUI

Dear all, I need to batch edit existing various 952 subtags, but *not* 952p Barcode (which would corrupt circulation I presume). I am not a Linux techie, and want to use the Tools feature, "MARC Import (managing batches)" but have been unsuccessful thus far. We use a matching rule that matches against Koha biblio, 999c (my previous question to the newsgroup). I've tried a variety of settings on the import, but always receive, "Number of items ignored because of duplicate barcode : 25" FYI, on this occasion I'm trying to batch edit 952c Shelving location, to changed items from one shelf to another. Thanks for comments, Nelson implementing a union catalogue in Cape Town, South Africa