I can't help but think that you are in the minority... and does seem strange the seat dupe passenger wouldn't want a business class seat rather than economy

my initial thought is they perhaps thought the middle seat was still in economy?

I have had a couple of unwanted upgrades, when traveling with friends who remained in coach.

always solved by offering my biz class boarding pass to the passenger sitting next to my friend. if necessary a quick word to the crew just to explain. never been a problem...