Duplicate entry not allowed in excel -answer to preet makkar

Dear Sir <br> i want to make an excel sheet in which i want that duplicate entries should not be allowed. <br> For Example in cell A1 if i write a name Raj <br> and again in cell say D5 i write Raj, it should not allow the entry and give an error message "Duplicate Entry"... Is it possible?

Answer: See attached file (A simpel answer to it)