My opinion on boxed gifts is my own and no amount of rationalizing the utility of cash will ever change my mind. If one is so concerned with not getting duplicate gifts than they should ask to not get ANY gifts at all as opposed to specifying that their guests give them a cash gift.
Again, my personal philosophy is that a guest is invited to help celebrate your wedding, not to help the wedding couple furnish their home, pay for their car/downpayment or honeymoon.
But given the number of threads opened on this topic over the years, I’ve accepted that others think differently and so it is to each his/her own shrugs