Sin ver ejemplos de la salida, difícil de decir. Pero, ¿ha intentado exactamente la misma consulta con un simple DISTINCT? agregado a SELECT ?
SELECT DISTINCT itpitems.identifier, itpitems.name, itpitems.subtitle, itpitems.description, itpitems.itemimg, itpitems.mainprice, itpitems.upc, itpitems.isbn, itpitems.weight, itpitems.pages, itpitems.publisher, itpitems.medium_abbr, itpitems.medium_desc, itpitems.series_abbr, itpitems.series_desc, itpitems.voicing_desc, itpitems.pianolevel_desc, itpitems.bandgrade_desc, itpitems.category_code, itprank.overall_ranking, itpitnam.name AS artist, itpitnam.type_code
FROM itpitems
INNER JOIN itprank ON ( itprank.item_number = itpitems.identifier )
INNER JOIN itpitnam ON ( itpitems.identifier = itpitnam.item_number )
WHERE mainprice >1
