Remove duplicate errors from `dab validate`

[abhishekkumams]

When i run the cli using the config file created in this test, i see duplicates of the errors Aaron mentions. While I understand that the errors may be sourced from different places, this makes the errors pretty noisy if there are duplicates..

• "Table Definition for Book has not been inferred" -> appears twice
  

Originally posted by @seantleonard in #2100 (comment)