Q: Uniqueness of my study if there is an existing study related to my research topic?
If the aim and method of the study on which I am working is similar to a study that uses the same aim and method, then can I do this study? If yes, what is the uniqueness (which may be asked by Editor of a journal)?
Before you finalize the aims of your study, please scan the recent literature relevant to your research topic to ensure that you are not repeating something that has already been done (a good place to start would be the most recent review articles on the topic). Think about why you want to have the same aim as that of an earlier study. The aim might be the same if the previous study could not fulfil it or if you need to add something significantly different from the findings of that study—perhaps a deeper unraveling of what has been uncovered on the topic. Alternatively, the aim could be similar if your study involves a completely different setting and/or different research subjects/objects. However, if these aspects are very close to those of a previous study, you might need to modify them accordingly.
Coming to the methods, it is common to use published methods or approaches. You might need to adapt or modify some elements depending on your study setting, number of subjects or treatments, analyses, etc.
Finally, remember to follow best practices of citing the previous study and avoiding plagiarism.