Thanks a lot Mr. RichardM, it solved my problem.

"RichardM" wrote:

Hi Shahzad

Select column A and go to Validation (on the Data menu)
select "Custom" in the "Allow" box and enter the formula:

=MATCH(A1,$A:$A,0)=ROW(A1)

then click OK

Any unique number is now allowed in column A but a duplicate will give
an error

Shahzad Zameer wrote:
Dear All,

I want that in a Column a number should not repeat again. Suppose if I write
in Column A a number 7 then it can't be write again in same Column A. Please
help how to do this?