It's a fact that people do that so the artist will get more money. However, it is NOT reflected on the iTunes charts, nor do I think iTunes reports multiple downloads to Sound Scan UNLESS it's been gifted. So, multiple downloads is not why Taylor is so high. There are actually that many more first time downloads than the #2 song, Justin Bieber. The popularity bars also take in account the ratings, but not so much as obviously we've had 2.5 star rating songs at #1.
Also, on iTunes 9, you are not allowed to download purchased music multiple times.