Does the highest mark always count?

This is essentially a two-part question, so I'll separate it:

1) I'm resitting some AS modules I did last year, I already resat them in January and got the mark I needed for uni, but I decided to resit them again (so I'll be taking them for the 3rd time) to try and get the highest UMS marks so in the case I mess up my A2 coursework or exam I'll have a safety net!

So my question is, if I mess these re-resits up, will I get to keep the higher marks I got in January when they give me my overall grade in August?

2) I've often heard people say, "you always keep the highest mark", the reason I'm confused about this is that my firm university has insisted I resit my exams from two of my A-levels last year in order to get the same grades as I got last year. If you always keep the highest mark, wouldn't I still keep my original grade even if I completely failed this year?

So confused, any help would be amazing