How does the cell get its information? From a formula or by typing it
in?
If it is by a formula then you could use the COUNTIF function to
determine if the value already exists in previous rows and return a
blank if it does. If it is by typing it in, then you would need to set
up data validation on each cell and set up a similar formula to
disallow repeat entries.
I realise the answer is a bit vague, but you have not given many
details in your post.
Hope this helps.
Pete