I use one-step for my Amex account and 20 times out of a month it works fine, only provide new activity. I'd say about once a month though, I get the last 10 or so transactions, even though most of them had already been downloaded. I just manually delete them rather than accept all as is my 95% of the time practice.
I don't think American Express has quite committed enough processing power to keep up with online banking demands. I have significantly better luck with my Discover and Visa accounts where I don't remember the last time I had transactions that were double reported.