heya, I am new here myself, so i hope its ok to post a reply......
Personally i think i would use Data Validation
Highlight the column you want to 'validate' and then choose Data>Validation from the tool bar
in the settings tab, choose custom and then type into the formula box
with "A" being whatever column your data is actually in
on the Error Alert tab, you have the choice of not allowing duplicates at all, or giving the user the option to override and accept the duplicate...
HTH, (and I hope I have understood correctly what you were asking too)