ConfigurationMetadataAnnotationProcessor can add duplicate entries #23916
Description
I've had builds fail recently due to duplicate meta-data entries. The most recent being https://ge.spring.io/s/p5yonxuszztr2/failure#1.
It looks like it might be possible to have duplicate entries in org.springframework.boot.configurationprocessor.metadata.ConfigurationMetadata, but I'm not sure how/why this happens. I wonder if the flattenValues method needs reworking.