I have two similar term importers set up. They import to two different vocabularies and both are using GUID as unique identifier.
Problem is that the TermProcessor is not checking the GUID corrctly. The function existingEntityId(FeedsSource $source, FeedsParserResult $result) only checks on feeds_item.entity_type='taxonomy_term'.
It should also check on feeds_item.id='[importer ID]'.