I thought it wouldn't let you send twice to the same address? I thought there was always a big warning "Do not send anything but this amount to this address," blah blah blah... maybe I'm remembering a different BTC service though.
Mt. Gox allows you to use any of the disposable addresses they hand out, for as long as you like.
It's a very good safety mechanism
(and necessary if you want to mine & track stats on a pool like Eligius, where your BTC address is your username)
Though every time you receive funds, you will get a new address. You can keep using any of your older ones.