Could you use a countif formula?
eg in [B1]:
=countif(A:A,A1)
would give more than one if you had a duplucate of A1 elsewhere in colum A

"ShamsulZ" wrote:

how to know that I'm not key-in the data twice or more in one worksheet? In
my daily job, there is thousands of data and the data cannot duplicate.
Thanks a lot friend.