do (someone or something) an injustice
idiom
                                
              
          
                                                      : to treat (someone or something) in an unfair way                                      
              
                             
They did her a great/terrible/grave injustice by not allowing her to file a complaint.
                                       I think you do the book an injustice when you call it "trash."
                         
                
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  Merriam-Webster unabridged




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