I was hired at my current workplace just over a month ago. During the interview process, I found out that someone I knew from university was also interviewing for the position. I'll call him Jim. Jim and I have never been close, but are merely aware of each other. Based solely on technical knowledge and relevant expertise, I believe that Jim would have been the better candidate for the job. In particular, he previously worked at a well known company in a position very similar to this one. I was therefore somewhat surprised when I received the offer.
On multiple occasions, my manager has made statements along the lines of “You'd be good at this project due to your experience with [some technology]”. I have only limited knowledge of the technologies mentioned, but I know that Jim has experience with them. Each time I've simply brushed it off.
More recently, I overheard my manager mention Jim's previous employer as though I had worked there. He's made this mistake twice, referencing the same company.
Given the above and some other hints, I believe my manager mixed up our résumés and may have hired me by mistake. The interviews were over the phone, so he would not have noticed by my face that I'm the wrong candidate.
I want my manager to know my actual background and areas of expertise and feel guilty that I “stole” a position using someone else's credentials. However, I'd also like to retain my position, if possible. How can I tell my manager that I was hired by mistake?