Hello there.
So what if a card with a duplicate is taken under control, should the duplicate also go under control or it would be discarded (because controller doesn't own card in order to use additional copies)?
There is what I find in RR(p. 7):
"A player may use additional copies, by title, of any unique card (indicated by a “󲀋†symbol in its title) he or she owns and controls in play as duplicates."