I like to use conditional formatting when it's convenient:

If you have invoice numbers starting in Cell A3, try conditional formatting
from A3 through as far down column A as necessary with this:

Formula is: =COUNTIF(A$3:A3,A3)1
Format: Pattern: Bright Red

Using that, the first entry of an invoice number is ignored, but all
subsequent duplicates are flagged.

Does that help?

Regards,
Ron

"Kenya" wrote:

I am working with a spreedsheet incorporating invoice numbers. How do I
create a warning when i repeat an invoice number