Today I've been getting "Error: An invalid or duplicated request has been submitted." about a third of the time trying to make valid buy/sell orders (that have enough shares or BTC to cover them). Often happens if make the buy/sell order after cancelling a previous one.
Yes, this has happened to me constantly. After I cancel an order, I have to reload a fresh page in Chrome, then I will be able to post another order. Some bug here.
Also, I have requested that server time be posted somewhere on the top of the page, or users are given an option to set times to display in their local timezones...
hopefully these suggestions help =)