Ah, yes, I know that message as I have had this same situation occur before. I started looking directly in the cmdb_ci_endpoint table as well as the extensions which would include cmdb_ci_endpoint_http which is likely where your entrypoint is and hitting the business rule that disallows the dupes.

IIRC, I would just delete them and re-add them. You may need to wait a moment while the delete runs the logic that cleans up and then re-load the page where you are trying to re-add it.