When adding new heading(s) (author, subject or series titles) to an existing bibliographic record, duplicate authorities entries are created for existing headings. New heading to the bib. record is not duplicated in authorities. For example, additing a new author only to a bibliographic record, existing subject headings on the same bib. records are duplicated in authorities. Have to manually remove those duplicated authorities with zero (0) bibliographic link.
Please, check the Linker options. If you already had duplicated authority and the LinkerModule option is set to Default, there will be no match, so a new authority will be created. If there is exactly one matching authority, it will be found and linked. Is it not you case...? You may also try with LinkerModule set to First Match or Last Match to verify if this is the case.