Not Entering the Same Number Twice

I'm creating a large bill of materials and need to assign part numbers to all components. I'm using several worksheets and want to avoid accidentally using the same part number twice. Is there a way Excel will tell me if I've entered the same number twice?

Thanks, Joel

Tom-

Can you explain how this conditional formatting is done? Will it work across several worksheets?

Thanks, Joel

Marcol-

This is great. Is there a way to simply give a warning instead of completely disallowing the same value to be entered. There are some duplicate parts in the BOM. Can this method work across several worksheets?

Thanks, Joel

I figured out how to just show a warning. Is there a way for this data validation to work across several worksheets?

Thanks, Joel