What can lead to Duplicated or Missing documents?

Hi,

I am trying to understand the feature Check Document Numbering under Administration->Utilities menu in B1 9.1

When I run this utility, I get a message in the status bar: "No duplicated or missing documents found".

This leads me to the question what can cause duplicated or missing documents in the first place !

Can you tell me some steps or scenarios how can I create the duplicated or missing documents, so that this utility can show me some records.

Thanks.