Edit: Since teamwork has come up in the answers, I think it would be useful that I add this. The other student has requested to team up with me, and the advisor has rejected this. Besides, I already have the solution to my research question. Not much left for teamwork on that section. I will have to share my solution with my supervisory panel, and then start the evaluation which I can't and rather not to do in secrecy.
A student recently moved from another area to study with my supervisor. Along with this change in supervisor, the student has also changed direction and has started working on the same problem as mine.
The change in subject is quite a sharp turn too. For example, think of a student with a background in microbiology(all the way up to masters and over one year work of their PhD), suddenly starts working on a machine learning model for aggression in dogs. While I have deidentified the subjects, the problem I have been working on is even more specific than the imaginary dog problem. It might be worth adding that the subject of my study was proposed by myself after over two months of literature review.
The change sounded weird to me at first, but I wasn't worried until I saw part of my study plan and one of my ideas in this student's review presentation.
I am sure that this has been my big mouth not keeping what I say in check, primarily because I usually think very positively of people. I can keep my mouth shut from now on, and the supervisors I have are quite good. However, I imagine they have quite a challenging task in remembering which idea/solution was shared by which one of their students, and not to talk about it to the other student unintentionally.
I am not sure why the hell someone would throw away years of background and start working on something that they have no background in and is already being studied under their nose. A tough challenge is that how do I exchange information without a worry that these ideas might leak out? One of my supervisors is a very busy person, I am sure that this person has a fantastic memory. However, is it possible that they forget which idea was shared by who? and unintentionally share it with the other student? What should I do? Suggestions?
To my principal supervisor, if they happen to see this: Please do not take offence, I have great respect for you and will always be grateful for all the incredible support I have received. This issue is part of my learning process, and I'd like to tap into all resources available to me so I can tackle and learn about it to the best of my capability.