Yes, Beatrice you can. I hope my article on Publication helps you here is the links below:

https://medium.com/write-a-catalyst/my-first-medium-publication-submission-process-b2a8c981e45e

https://medium.com/write-a-catalyst/how-to-find-join-a-publication-on-medium-6d56c1bd96b5

as for the Second question the answer is "Yes" I believe you will have to do that so you don't get notified for having duplicate articles. I hope this helps and it is exciting that you are thinking about joining publications. I find it very supporting and wonderful experience. 😊