fixes #90, avoid trying to re-add prefixes that were already added
fixes #90, avoid trying to re-add prefixes that were already added

OpenTriply/YASGUI.YASQE,OpenTriply/YASGUI.YASQE,OpenTriply/YASGUI.YASQE,YASGUI/YASQE,YASGUI/YASQE,YASGUI/YASQE