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I received the same offer from Cornell today that I have from WashU. Do you think this could be used as a bargaining tool to bump WashU's package? If so, should I begin negotiations now or wait. I'm signed up for the Feb test and hope to score 170. I scored 168 in September which was around my PT average but recently have been drilling nothing but LR and just scored a 175 on my last PT, albeit not perfect test conditions and I had taken the same PT when I was studying for September. Should I wait till I have my Feb test back to begin these negotiations or start now mentioning the Cornell offer and then later on if I score higher mention that as well?

You can always start negotiating once you have offers in hand. It won't preclude you from negotiating more down the line.