One of the questions that we often get is "How do I know when to stop searching?" It can be frustrating to seemingly never get to the end of your literature search.
An easy guideline to use is this: if you keep seeing the same references over and over, chances are that you've exhausted your search. What you're seeing is that your research topic has reached critical mass and you've found all the existing articles that are relevant. You can stop the retrospective searching and focus on writing your literature review.
You should also check the Keeping up with the Literature tab to see how you can be notified about relevant research on your topic as it becomes available!