I am working on a master-thesis research paper. Recently, I have chosen a research topic and found a related survey dataset to work with. However, just now, I have found out that other authors did the same work for the same country, but used a different survey and for years earlier. Now, I am confused about what to do. If I would carry on the research, would it be considered as a plagiarism, even though I will use another survey for later years, and work in a different setting?
Plagiarism is selling somebody else’s idea as your own. Therefore as long as you properly cite the other paper, you are in no danger of committing plagiarism. The worst that can happen is that you are reproducing an existing study¹ instead of doing something novel. Whether this is a problem, is for you and your supervisor to decide, as you know the differences in detail.
On top, suppose you hadn’t found out about the other study: In this case you would not have committed plagiarism since you cannot possibly use the ideas of others without knowing them. Had you published your paper in this case, you would perhaps² be guilty of sloppy literature research, but not of plagiarism. However, the evidence may (incorrectly) point to you being a plagiarist and thus you still want to avoid this situation.
¹ which scientists almost universally agree on being done too rarely.
² if the other paper is sufficiently easy to find
Reproducing a published study is good science. It isn't plagiarism as long as you properly cite the earlier work and don't copy from it.
It is especially good science if you have any doubts about the conclusions of the earlier study or about its applicability in new circumstances.
But, to be published yourself, you will need to be clear about why you are doing this and what, if anything, is new and different in your study as compared to the old. That isn't an ethical issue, but just the fact that publishers want to publish the new and novel.
If your methodology is the same, you need to say that and cite the original. If you have made changes you need to carefully explain them along with the reasons for the changes. If your conclusions are different you need to analyze why that is and explain it.
But it can be good science. There is quite a lot of published science that isn't good. Often and especially if it was done by someone with an agenda beyond seeking the truth.
The answer of Wrzlprmft gives a good explanation of plagiarism and how to avoid it.