Bug 20421 introduced CheckPrevCheckout logic when placing a hold on the staff client. It would be useful to have the same CheckPrevCheckout logic implemented for patrons when placing a hold on the OPAC.
I had a patron request for this functionality recently. If it does make it to the OPAC, it would be good to add the opt-in/opt-out toggle in the OPAC as well so patrons can turn it off or on themselves.