Depends. If it's a single resit they don't mind (my girlfriend did that and she's at Cardiff med school). Other Universities really do not care for it, especially if it is a trend. If it's a blip it won't cripple a decent application. Just focus on a rounded application, e.g. good work experience, decent grades (even if a resit is required), and extra-curriculars. Having said that you may wish to be tactical, e.g. Oxbridge might not be the best idea with serial resitting.