Cross-sheet data validation: Allow only unique values to be entered

Hi guys,

I need help with the following issue.
Excel supports uniques to be recognised with the formula =countif([range],[cell in question])=1 and is able to show a notification when the specified range is in the same sheet using data validation.



The validation does not support crosspage unique checks. Is there anyone who knows how to achieve this?

I'm personally not familiar with excel, but I have some programming xp.

Thanks in advance

I've got it to work, if someone gets into the same issue, here is an example with 30 rows checked for uniques in 12 sheets per month.
I'm using named ranges, to keep my formula short,

However keep in mind, the data validation does not work for pasted values. To make these pop up I have added some conditional formatting.