protected PrincipalAttributesCoreProperties.MergingStrategyTypes determineMergingStrategy() {
        return ObjectUtils.defaultIfNull(getMergingStrategy(), PrincipalAttributesCoreProperties.MergingStrategyTypes.MULTIVALUED);
    }
The merging strategy that deals with existing principal attributes
and those that are retrieved from the source. By default, existing attributes
are ignored and the source is always consulted.
core/cas-server-core-authentication-attributes/src/main/java/org/apereo/cas/authentication/principal/cache/AbstractPrincipalAttributesRepository.java