How to avoid plagiarism when you come up with an argument for a class paper and later discover that another author has the same argument?
From your description, it doesn't sound like a question of plagiarism (you say you're referencing the relevant paper, and I assume you're either paraphrasing it or using quotation marks as appropriate), but a possible lack of novelty. Lack of novelty is more of an issue when you're trying to publish in a journal or present at a conference. Usually class assignments don't have to be novel; they just have to reflect your own work. If your professor is satisfied, I don't think you have a problem. I would do my best to extend the ideas from the other paper, i.e. add something to their analysis.