I took 8 courses in total this past year. I took a course first semester and didn't end up doing so well so I retook it second semester as well. I passed both times. So I basically took 7 different courses.

When calculating my average, do I add all my marks and divide by 8? or do I just use one grade for the course that I repeated?

You take no. of credits the course was worth * the grade point you got for that, Add all of THOSE and divide by the total no. of credits, that is, 8*3 for you, I'm assuming. You also include both the grades you got for the same course. I think so.

when they release the final grade reports, your cumulative and sessional averages will be calculated for you anyways