I have a spreadsheet with columns X, Z, AB, AD, AF and AH labelled with the
following column headings: Cheque Number 1, Cheque Number 2, Cheque Number 3
etc.

How do I set Data Validation on these columns to do the following:

1. Only a except a 9 digit Cheque Number.
2. To not allow any duplicate entires in any of the columns listed above.
For example; a value entered into Column Z will be cross referenced with
values in Z, AB, AD and AH to see if it has been duplicated.

Is this possible?

If not I would be quite happy with just duplicate values to be located in an
individual column only. For example, for the validation process to search on
a single column only.

Kind regards,

Simon.