A user marked one of my questions as duplicate, I don't really understand if I have to do something about that or what ???
Are you referring to this one: How to download and install java plugin for firefox 19.0
If so, I am then the evil man that marked that question as duplicate.
Is not a bad thing and you do not have to do anything else. It just means that there was an already existing and answered question similar to yours on the site. The question that was used as the original was How do I install Java?
Which answers your question (In several ways made I add thanks to users like Bruno Pereira, Lekensteyn and others).
So do not worry about it. This is one of several methods we do here to have a neat and clean site that users can use to find the questions quickly without getting into confusion when they see several similar questions (Eg: Like looking for an Nvidia problem and finding 10 similar questions). Is not something bad you did, you might have just tried to search for the question in the beginning but could not find one or at least one that solved your problem.
First check the master question and see if it solves your problem. What is the master question? Near the top of your question there will be what we call a "banner" pointing the the master question.
After looking over the answers to the master question, see if they solve your problem. If so great! Your question will now help point others to the correct answer.
If the answers to the master question don't solve your problem, you'll need to get it re-opened. First, flag it for re-opening, explaining why your question isn't a duplicate. It also helps to edit your question to include why the master question doesn't solve your problem.
Last but not least, ask a question about it here on meta! It is perfectly acceptable to post a question here asking for your question to be re-opened. This is ususally the quickest way of getting it done, as users with 3,000 or more reputation can vote-to-reopen your question. It takes 5 votes to re-open a question, but moderators can always close and re-open a question on their own.
Once you have 250 reputation, you can view the amount of close or re-open votes your question has.