Display distinct / unique values in lookup field

Display distinct / unique values in lookup field

Hi,
I have the following in Form1
 Author
Book title



Sometimes, i have more than 2 books submitted by the same author
ie --
  •  Au_ABC --> Book_abc
  • Au_123 --> Book 123
  • Au_ABC --> Book_a1bc


In form 2 i want to use a lookup field to pull the author name  from form 1
Nomal lookup displays

  • Au_ABC
  • Au_123
  • Au_ABC
..
where as I want the list to show
  • Au_ABC
  • Au_123

I am not sure what to give in the criteria filed.
Please advice
Thank you,
regards

Deepa