Try this:

Assume range of data in column A is A1:A10.

Enter this formula in B1 and copy down as needed:

=IF(COUNTIF(A$1:A$10,A1)1,1,"")

A minor change the OP may want to consider is this...

=IF(COUNTIF(A$1:A$10,A1)1,COUNTIF(A$1:A$10,A1),"" )

which will show the total number of repeats (if there are 2 or more) instead
of just a 1... having the count may make whatever the OP is doing with the
duplicates easier as he will know how many need to be processed in total.

Rick