Hi
Select the range for your list (p.e. A2:A100)
From Data menu, select Validation
Allow=Custom
Formula=(COUNTIF($A$2:$A$100;$A2)<2)
(this formula is for my example range of course - you have to use your own
range here)
You also can edit error messages for dubblicate entry here on Error Alert
tab.
Press OK when you are finished.