Unique records when we are using developed attributes for categories.

Viktoria_Svensson_(sablinuser2_-_(deleted))

We have our own developed attributes for a category. We've named them "Registration number", "Status" and "Issue number". The combination of these three attributes have to be unique, which have to be controlled when a user adds a document with these attributes in a category.My question is: What is the best strategy to take for a customization in order to do these controles? we aren't allowed to have duplicate of a specific combination of Registration number, Status and Issue number.Are there any callbacks, events or something to use?Regards,Viktoria Svensson