This is part one of two, Re-dos and re-takes are a big time sucker, for both teachers and students, but we need to find a way to do it. Providing the opportunity to recover form a bad grade is essential to keeping the students engaged. Give them hope to succeed!
In real life all kinds of high stakes tests (driver's license, bar exams, nursing boards, etc) can be re-done. "How pompous is it for a teacher to say MY TEST is so important you only get one chance at it!"