Actually...would this happen? As Its About Time showed that two instances of a time traveler can exist simultaneously... Prime!Twilight made the decision to blow off this party, because she was enroute to Ponyville under orders from Celestia.

Traveling!Twilight goes where the original is not, basically the timewarp version of what Pinkie was trying to do with the mirror pool. There's a paradox crash when you get to this episode, of course, but not so early.