The title "Óðinn Riding Sleipnir" was showing up twice in the list of titles, even though the list is created from distinct values with space normalized. It turned out that character 2028, an invisible line break character, was present in two of the three documents concerned, and normalize-space does not seem to remove that character, so that was the cause of the duplicate value. PAB will fix the source XML. Noted here for future reference.