Prevent duplicate entry

Hello All,
This might drive some of you nuts...so excuse me I'm new to this.
How do I prevent duplicate entries across worksheets in a workbook?
The data validation seems only to do one or two sheets as a range search....thanks.

Hi

Can you give an example sheet please. It is often easier to see the issue on a real sheet. You can upload the example using the Go Advanced link when posting

Thanks for the response. Here the sheet in question!

The column to monitor is "C"
Thanks