I) To prevent duplicate values (from A2 to A10) when entering data:
1) Select the range (eg: A2 to A10)
2) Data -> Validation -> Settings -> Custom -> Formula:
=countif($A$2:$A$10,A2)=1
II) To find duplicate items in combine column:
To check whether the data in column C is Duplicate or Unique:-
Firstly enter values in column C,
then enter the formula in column D:
=if(countif($C$1:C1,C1)>1, "Duplicate", "Unique")
then copy the formula downwards.