private void validateEndpoints(List<Endpoint> endpoints) {
        Set<String> endpointIds = new HashSet<>();
        for (Endpoint endpoint : endpoints) {
            if ( ! endpointIds.add(endpoint.endpointId()))
                illegal("Endpoint id '" + endpoint.endpointId() + "' is specified multiple times");
        }
    }
Reads the content of an (implicit or explicit) instance tag producing an instances step

@param instanceNameString a comma-separated list of the names of the instances this is for
@param instanceElement the element having the content of this instance
@param parentTag the parent of instanceTag (or the same, if this instance is implicitly defined, which means instanceTag is the root)
@return the instances specified, one for each instance name element
config-model-api/src/main/java/com/yahoo/config/application/api/xml/DeploymentSpecXmlReader.java