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Dean would definitely be harsher, but he wouldn’t kill Sam right away (one of the times it’s really, really good Sam was found by Dean and not by certain other hunters).
He would likely close his hand around Sam much tighter, though, and confiscate the knife from him before anything else. If Dean takes the time to look at the knife, there would be a lot more shock and demands of where did you get this and Sam completely confused and winded by it all, likely ending up under a vase until they both realize why they have matching knives, and then guilt for the ages for Dean.
Brothers Apart 