When submitting a list for each round, you acknowledge that you are not required to fulfill any act for anyone if it is not in you power (but it would be really awesome if you did.) Thus you must also acknowledge that no one will be forced to fulfill your act if it's not in their power to do so (but it would be really awesome if they did too.)
If you see something that you would like to fill but has already been filled, don't let that stop you from filling it again. Multiple fills are much appreciated (the more the merrier.)
Once a round closes and a new one opens, you may still fulfill older acts.
Even if you do not post a list, you are more than welcome to fulfill anything that strikes your fancy.