Can I use the same model more than once in the case? A model may be suitable for question 1 and question 4 for instance. Is this okay or would be termed repetition?
It is absolutely fine to use a model more than once for different questions provided it fits the questions. However, take care! It means that the examiner has constructed a paper which requires or allows the same model to be used. That would be an odd exam. Why test the same thing twice? So be wary. Is the same model really the best one to use in both answers?