Select the range of cells in which you want to prevent duplicate values.
Choose Validation from the Data menu. In Excel 2007, click the Data tab and choose Data Validation from the Data Validation option's drop-down list (in the Data Tools group).
Click the Settings tab.
Choose Custom from the Allow drop-down list.
Enter a formula in the following form into the Formula control:=COUNTIF($range,firstcellinrange) = 1
Click the Error Alert tab.
Enter the text Duplicate Entry in the Title control.
In the Error Message box, enter a meaningful description, such as The value is a duplicate value and therefore, not valid.
Click OK.

I hope this is helpful!
Tim Paine