Collaborative filtering when multiple items are rated multiple times by same user
I don't think there is any academic work on the subject, at least that I know of. 

One simple way of using that data would be to use the mean of the ratings or other average like measures such as a moving average, a time weighted average, the median, etc. 

But this approach is probably not exactly what you're looking for.

Try to look at collaborative filtering approaches with temporal dynamics, there might be something interesting for you.