My master's thesis is extremely similar to an article that had already been published. Is that okay? My thesis is related to hispanic literature and culture. After doing a fair bit of research I formed my thesis and now as I am in the process of writing my literature review, I am realizing that my thesis is very similar to an article that I have read. The papers differ in that their theory is only applied to the events that happened in one country and I am applying a very similar theory to about 10 different countries throughout Latin America. I cite their work throughout my paper and plan on including it in my literature review. Is this allowed? Or is my thesis no longer valid/ acceptable because someone else had the same ideas applied on a much smaller scale? I do not intend to publish my work in a journal or other publication, I just need the paper to graduate.
I had this same question when I began writing my concept paper for my dissertation. I was alarmed when I found similar articles related to my dissertation topic and I almost changed topics in a panic. My advisors told me that finding related research was to be expected, but that I had to prove that my research would add value that was missing or limited within the findings of the prior studies. To provide proof, I created a matrix of all of the related material from my literature review to show how the others sources were similar or different to each other and how my research would fill the gaps. 

I think it is very important to show that you are aware of the related research and that you have a clear understanding of what you are adding. One place to get a clue of this is to look at the Recommendations for Future Research that will be in the closing of the related research.  

Good luck with your thesis.