conditional formating might work using
select column with invoices (using A in this example)
<format<conditional formating
set equation is =sumif(A:A,A1)1
and format as wanted

"Rob McEwan" wrote:

I want to set up a spreadsheet for a novice, and careless, user to input
details of invoices to be paid e.g. invoice number, supplier, amount etc.

From past experience this person will submit an invoice even though it has
been passed for payment previously.

I would like to find a way to mark/identify duplicate invoices, or even just
indicate that the list includes a duplicate, within a list e.g. where one
cell in a column is exactly the same as another cell in the same column.