my opinion is you should definitely tear it off and redo it. you could go right over the top again, but depending on what you have up there (shingles, tar/gravel, rubber) and what kind of roof it is (flat, pitched, etc.). if you keep going over it, you'll end up with so much more weight and more to remove when you do get a leak and have to patch it, that it'll start to get really bumpy.
if there's no leak however, don't worry about it yet - unless you're selling (inspector). :-)