Take a look at the Conditional Formatting option at

http://cpearson.com/excel/duplicat.htm

Also consider

Preventing duplicates on entry:
http://cpearson.com/excel/NoDupEntry.htm

In article ,
"JD" wrote:

If I type the same number in the same colum say ( D )
is there a way to flag ,make a sound,and or change color
in that cell that i have entered a duplicate number