title: Before create callback, how to validate if duplicate record exists and if so return?
body: <p>Before a record gets created, I want to validate that a similiar record doesn't exist.</p>
  
  <p>When I say similar, I want to compare 3-4 columns to see if they have the same value, if they do, then don't save and report back a validation message.</p>
  
  <p>This only occurs on creation, there might be duplicates during updates etc. but during creation I don't want to allow duplicates.</p>
  
  <p><strong>How can I do this? i.e. which callback is best for this, and how to add to the validation errors collection?</strong></p>