Some fest are a bit picky, though.
One of the really big ones has a policy like that... film has to be a premiere at that fest, for one of the prizes.
Could someone explain the premiering your film on one festival means you can't on another.
This will vary from fest to fest. Some won't care; others will.
Make a list of the fests you'd like to enter your film in (because there's no way you'll be able to afford to enter them all) and when you have your list... read up on their policies. It may mean juggling about your submission schedule, if you need to be a premiere at a certain prized fest.
Might be time to get that book by Chris Gore, as well.
No idea on the question about entering a script for a screenwriting contest, then making film later, and how that would affect eligibilty.