Nodes are duplicated, not replaced

We set up a feed importer to import XML data from an uploaded file. This GUID is set to use the record ID from the XML file, GUID is set as unique, and the Node Processor is set to Replace Existing Nodes.

We loaded a small subset of the records, but when we load the whole set of records, which includes those already loaded, the importer duplicates the existing nodes instead of replacing them. Am I missing a setting somewhere? Is there something else I needed to do when setting up the node processor? Is this a bug?

Thanks for any insight you can share!

Laura