Hey,
I'm doing A2 atm and have been through the 'not so good AS results' moment last summer.
It's very easy just to brush off your AS results, even if they are particularly bad, because you have a whole year to rectify them.
In answer to your question, yes of course you can resit - obviously at a cost and at sacrifice of January time that is usually used for A2 mock exams. However, if it's necessary you will have two different chances to resit (january and may/june). Clearly, though, it is better to do well the first time round.
If I had a message to anyone doing AS levels at the moment it would be - REVISE! It's easy not to worry too much but it's much nicer to be able to present nice grades to universities come November when you're sending off your application.
Try not to worry - just do your best. Always a chance to do better next time!
Unless you're going into medicince... then you're stuffed.