No one is going to be able to give you a concrete answer. Specifically in regards to Cambridge, does it look good to have to resit? No, of course not - but it doesn't mean you can't get a place. The problem is, in my opinion, doing well (even very well) in mathematics/further mathematics at A-Level does not mean you'll be a good mathematician. The skills needed to do well at A-Level differ massively from the mathematical skills needed to do well at University - which is partially why Oxford/Cambridge make people applying sit other exams and have interviews (which are vastly different to the kind of work you do at A-Level). The reason I'm saying this, is because I would say the tutors at Oxbridge look far more at your 'other non-mathematical' A-Levels than is perhaps logical, because this gives a good indication of working under pressure and coping with a workload - you'll be surprised how massively varied mathematics is at University. Unlike A-Level maths where generally the maths is the kind where if you're good at one part, you're good at near enough all of it, at University the maths is considerably more varied.
If you intend on applying after your AS-Levels then the resits will have more of an effect than if you applied post A-Level (but I guess that is somewhat obvious). It's also going to be dependent on how you perform in the summer, really unless you've got AAA in your AS-Levels the reality is your application will not be competitive whatsoever purely based on A-Levels. Some would argue that even if you did get AAA in the summer (and high As) it still wouldn't be competitive because of the resits - but that doesn't mean you can't 'wow' at the interview and get an offer because when it boils down to it, the real test is going to be sitting A2 exams and two STEP papers in the summer next year.