avoid duplicate URLs based on DOI, see ubtue/DatenProbleme#80, ubtue/DatenProbleme#83, ubtue/DatenProbleme#86
avoid duplicate URLs based on DOI, see ubtue/DatenProbleme#80, ubtue/DatenProbleme#83, ubtue/DatenProbleme#86

ubtue/ub_tools,ubtue/ub_tools,ubtue/ub_tools,ubtue/ub_tools,ubtue/ub_tools,ubtue/ub_tools,ubtue/ub_tools,ubtue/ub_tools