Although it should be noted that the butterfly tattoo does seem to change in size. I coulnd't find a clear image of it looking larger (though I'm sure I saw one earlier), but this looks bigger to me:
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Nick says things a couple of times, but he never actually spells it out. I havent' got time to go through the script at the moment to try to find the exact places or wording, but one of the times you cuddle with Jess on the sofa watching TV, for instance, he sees and says something like "Aw man, he got the good one", while after the blowjob at the start, Nick refers to her as "the clone of the devil".