Webform 5.x lookup validation via data table lookup I want to block/validate a new entry based on another submitted form

e.g.


If a site number has been used I want to block any new entry that wants to use that number?

Is this possible? 

If so How?

Thanks all
Webform elements support unique value validation, which I should be able to address your requirement. If not, you may need to write a custom form alter hook with a custom validate callback.