What is the chance of 2nd shoulder dislocation if you had 1st dislocation at 20?

An anterior shoulder dislocation is the most common form of dislocation and once it happens, you are over twice as likely to have it happen a second time and even more likely to have it happen a third time and so on.
Here's an article on shoulder dislocations that may interest you: http://www-cs-faculty.stanford.edu/~nick...

Oh, it is very common for this to happen. The ligaments can be stretched and it makes it easier to happen a second time. I've seen it many, many times

i dislocated my shoulder playing football, and later that season it dislocated again. so its pretty easy to dislocate it a second time.

Well, you're missing a few details.....

how old are you now? are you playing professional rugby or in martial arts classes? are you in roller derby or big time wrestling?

Yes, I'm teasing just a bit. If it was a freak accident that caused the first dislocation, well that's one thing. If an orthopedic doctor said that you have "rotator cuff damage" or you have weakened the joint connections at the shoulder, well, there's another story.

Does it feel more unstable than before you dislocated it at 20? Do you do strengthening exercises? All things to think about. Good luck.

It gets easier to dislocate each time you do it. The shoulder isn't a secure joint - it is a ball and socket, but it is held together by muscles and tendons and once they stretch, they don't go all the way back to what they used to be unless you do a whole lot of physio, and even then they can be less elastic than they used to be. So, yes, the chances of a second dislocation are pretty good.