Have them sign the card and move into a trick that doesn't use a dupe?
What effect do you have in mind, Queenie? I realise how annoying is when people say "if you're getting that response, you're probably not doing something something
blah blah blah blah..." because there are always people who, despite having zero idea how it's done, will just start with the verbal diarrhoea because they can't take not knowing... "it's two cards! It's a magnetic card! It's colour changing ink! It's a sticker that you secretly put on! It's a fake marker pen!" etc - and inevitably, sometimes it is
But - I do try to do tricks with dupes that don't *seem* as though they'd be done with dupes, rather than "card is chosen, pushed back into deck, magician whistles, three-legged dog runs up with "their card" tied around it's neck with ribbon" etc!
But, given it still will occasionally happen, see suggestion number 1!