Stopping Duplicate data entry

Hi, can anyone help with the following problem:

I have a spreadsheet that records all work received and tracks its progress. Each piece of work has an individual unique reference number. The problem I have is, customers often submit the same piece of work more than once and it ends up on the spreadsheet more than once resulting in duplication of work.

Is there a way of making excel flag up when data is input into a column when it is already there. I've have been playing with lookup tables and the count function, but can't seem to figure it out.

Any ideas?

Thanks

Hi,

To prevent duplicate entries with data validation...

http://www.contextures.com/xlDataVal07.html

HTH
Carim

Thank you so much, you've have saved me so much time.

Just to complicate matter further, what it the unique reference was split over two cells?

Could this be amended so it will not let the same combination of cells be entered twice onto a worksheet.

Thanks again.

Glad to see your problem is apparently fixed.

Regarding a unique reference split over two cells, your data validation formula should most probably use sumproduct() ...

Why don't you post a zipped copy of your worksheet, for a tailor-made answer ...

HTH
Carim