Assuming data is in column A:

Formula is: =COUNTIF(A:A,A1)1

"SGT Buckeye" wrote:

I have a column that contains 528 membership numbers sorted in
ascending order. Some of the membership numbers are duplicates and
each is 8 or 9 digits long. I would like to apply conditional
formatting to this column to identify the duplicates so I don't have to
go through and do this manually. Please help.