It sounds like you have seen a lot of GMATPrep questions already - and, unfortunately, that means you will almost certainly continue to see known questions if you continue to use that software. Re-downloading the software won't help; they haven't changed the question mix in a while and aren't planning to again until March or April of next year. :(
So that leaves you using MGMAT or other CATs rather than GMATPrep. One thing you can do - stop studying GMATPrep Qs and then in a few weeks or a month you can try another one to see. That does mean, though, that you'll have to avoid a lot of Ron's stuff and my stuff, since we're allowed to publicly teach GMATPrep Qs but not OG Qs. :(
Oh, wait - and I just saw your title that you're 3 weeks away. Okay, just forget about GMATPrep. You may want to do GMAT Focus a couple of times (
http://www.gmatfocus.com) just to make sure that you're okay on official quant Qs. GMAT Focus is a mini-quant diagnostic of 24 official questions. It's adaptive and it doesn't overlap with OG or GMATPrep. There isn't a verbal version, unfortunately.
Have you been taking your practice tests under 100% official conditions, including essays, length of breaks, etc? If so, then you look like you're in pretty good shape overall!
Here are a couple of articles for your last 2 weeks:
http://www.manhattangmat.com/blog/index ... game-plan/http://www.manhattangmat.com/blog/index ... to-review/