I noticed the same a few weeks ago when I accidentally set the same order twice and they both showed up on the orderbook. I just left them both up to see what happens and the second order was just a zero value transaction when it was filled.
I didn't toy around with it but I guess you could set up huge orders, then just move the XLM to another address and none of them would get filled. It shouldn't be a big problem to tie the assets up in the account similar to how the minimum for trustlines etc are tied up. So in case you open an order worth 1000 XLM, that XLM is tied up until the order is filled or cancelled.