=RANKEQ([Request ID]@row, COLLECT([Request ID]:[Request ID], [Item Number (SUPC)]:[Item Number (SUPC)], [Item Number (SUPC)]@row, Duplicates:Duplicates, 1), 1)
My best guess, without seeing your sheet, is that the problem is coming from the last "1" in your formula. That "1" should be defining a range, and it doesn't look like it would be possible. If you remove it, so your formula is just this:
=RANKEQ([Request ID]@row, COLLECT([Request ID]:[Request ID], [Item Number (SUPC)]:[Item Number (SUPC)], [Item Number (SUPC)]@row, Duplicates:Duplicates, 1))
do you get the intended result?
ref must be one of: categoryID, siteSectionID, category, category/categoryID, category/name, category/description, category/url, category/allowedDiscussionTypes, locale, siteSection, siteSection/basePath, siteSection/contentLocale, siteSection/sectionGroup, siteSection/sectionID, siteSection/name, siteSection/description, siteSection/apps, siteSection/attributes, layoutViewType, discussionID, commentID, page, sort, discussion, discussion/name, tags, breadcrumbs, discussionApiParams, serverDraftID, serverDraft.