dropped once, dropped again?

It's not a hard and fast rule but most sororities won't invite you back if they dropped you. Friends of sisters get dropped all the time. You have a better chance if your friends become highly respected/influential/involved members of the sorority. However I would trust that you were dropped for a reason, and keep a more open mind going on next year, because people really do usually end up where they fit best

Once dropped always dropped. That's what my sorority goes by.

I have friends in all 6 sororitys and I'm pretty sure that's the policy for all 6 sororities. Sometimes, even the ones who didn't drop you will be wary of your reasoning for going through recruitment again. Your best bet is to go through informal with one of the 3 sororities that do it.
I also have to warn you against becoming friends with people who got into certain sororities, because a lot of them will see right through that game and put you on the drop list going into recruitment, before you even walk into the room. It can seem very fake.
The only way you may not get dropped is if you genuinely turn around who you are, what you're involved in, your attitude, etc, without it seeming fake or like you're trying hard, which is nearly impossible.