While working on https://issues.apache.org/jira/browse/ARROW-2255 (serialize custom_metadata in the integration tests), we had the following discussion on GitHub: https://github.com/apache/arrow/pull/6556#pullrequestreview-372405940
In short, although in Schema.fbs custom_metadata is declared as an array of KeyValue pairs (so duplicate keys would be possible), all reference implementations assume it to represent an associative map with unique keys. Is there a use case for duplicate metadata keys? It seems that an acceptable resolution might be to note in Schema.fbs that implementations are allowed to assume that keys are unique Ben
