Many Thanks for your reply, but there is no source of confusion.
when you are using auto number, insertion of duplicate  record happens alot.
besides, please consider that such preventions should not occur in UI at all, because of the severe performance  degradation in searching DB for every inserting  record. also, as benchmarks say, PK-String-search in a db will never be as fast as numeric  search.
of course as i said, in smaller  application you can choose this strategy and even using Hash Algorithms with String output  (such as SHA-1and MD5) would be good. but in larger ones, only Numeric PK is recommended.