Jefffrey commented on issue #10160:
URL: https://github.com/apache/arrow-rs/issues/10160#issuecomment-4769309225

   the arrow spec explicitly does state that dictionaries can have duplicate 
values:
   
   > Note that a dictionary is permitted to contain duplicate values or nulls:
   > ```
   > data VarBinary (dictionary-encoded)
   >    index_type: Int32
   >    values: [0, 1, 3, 1, 4, 2]
   > 
   > dictionary
   >    type: VarBinary
   >    values: ['foo', 'bar', 'baz', 'foo', null]
   > ```
   
   
   - 
https://arrow.apache.org/docs/format/Columnar.html#dictionary-encoded-layout
   
   so this would be more a feature request, in wanting to deduplicate a 
dictionary for downstream performance reasons


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