How do you want to find out? You could use a helper column and use
=COUNTIF(A$1:A$100,A1)>1
And copy down, anywhere is says true you have a duplicate.
Or, you could conditionally format (format, conditional format) the first
cell using the same formula, and have the cell highlight in red if its a
duplicate. Then just copy the formatting to the other cells in your range.
