instance method deep_deduplicate

# File activerecord/lib/active_record/connection_adapters/schema_cache.rb, line 449
        def deep_deduplicate(value)
          case value
          when Hash
            value.transform_keys { |k| deep_deduplicate(k) }.transform_values { |v| deep_deduplicate(v) }
          when Array
            value.map { |i| deep_deduplicate(i) }
          when String, Deduplicable
            -value
          else
            value
          end
        end