I have 12 columns each with various amounts of data in them. I am trying to
see if there are any duplicates on the sheet at all so column B might have
data that is also in column E, and column A might have data that is also in
column L. I need a way to see which items are duplicates and highlight them
possibly.
I tried the

=if(countif(A:A,B1)0,"Dup","")

function listed in this forum, but it is not very efficient for the amount
of data I have, can anyone help?