Moneydance downloads all the transactions the bank offers. After
you go through and accept the ones with the blue circle, they
shouldn't download again. It takes a little bit to "catch up", but
once you do, you should only get the new ones. If you delete the
ones with the blue circle before you accept them, they will come
back on the next download.
Moneydance "learns" from how you categorize transactions over
time. It should get better, the more you correct the downloaded