Is it possible to loop back up to a previous question in a Sharepoint Survey using branching to allow multiple entries?

Sorry, but no. Surveys can only branch forward, not back. Mike Smith TechTrainingNotes.blogspot.com

Suppose you already have Question1 and Question2 in a survey, i guess you want the user to take Question1 again based on his answer on Question2, kind of asking the user to think again. I think you can create a new question, Question3, which ask the same as Question1. The advantage of having duplicate questions is you can track user's answers to the same question before and after Questions2. This will reveal more information. It may be better than let the user go back and modify his answer to the first question.