xrefwarn – Only one warning for an undefined citation

The pack­age patches the ci­ta­tion mech­a­nism so that you get at most one warn­ing about each ref­er­ence. (Without the pack­age, you get one warn­ing per ci­ta­tion, whether or not the ref­er­ence has al­ready been cited.)
Since the pack­age works on in­ter­nal macros, other ci­ta­tion pack­ages may not treat it kindly.