The file namingsystem-registry.xml (which is a Bundle) has two entries with the same id.

Both entries <fullUrl value="http://hl7.org/fhir/NamingSystem/us-ssn" /> and <fullUrl value="http://hl7.org/fhir/NamingSystem/us-medicare" /> in the bundle have <id value="us-ssn" />. Presumable, the last one should be "us-medicare".