Is it possible to duplicate the data base on Civi ? (contacts) Because i need to do it for my traineeship.I don't even know if it's possible but maybe does an alternativ way to do it. thanks and have a nice day.
There is no feature through the UI that lets you duplicate your whole set up since this also involves the CMS etc. Hence suggest you do what Christia suggested in the comments, namely
set up a local copy of your website and the CiviCRM by following the instructions. Then import a copy of your CiviCRM database.