At this moment, with the system preference CheckPrevCheckout, Koha will look at the patron’s circulation history to see if they have checked this item out before. It would be helpful if Koha could also check the customer’s current loans and provide a notification if attempting to lend a book that the customer already has on loan.

Of course, this need wouldn’t exist if the AllowMultipleIssuesOnABiblio setting were set to 'don’t allow,' but there are other requirements that necessitate keeping it as 'allow'.