The reason you have been contacted again is because the hiring manager felt that though you were not a best fit for the position at that stage, you have potential. It might be that you were well liked but eventually they found a candidate somewhat better than you.
As a hiring manager, I also interview 4-5 candidates per job. Sometimes I like candidates, but eventually find a even better candidate. So I put the next best candidate on hold rather than reject, so that should another position open up, I can consider them or recommend them to other hiring managers.
My take would be to go for it, approach it like a new interview, but at the same time, as suggested in the other answer, consider what you have said before, what sort of questions came in, and if you need to brush up on something which you didn't answer well last time.
Most people would not have bothered to call you again if they had no intention of hiring you. Another possibility is that the HR is really bad and they have no record that you interviewed earlier with this hiring manager. In that case, a polite way of approaching would be to as the HR, I interviewed with you 2 months back, what is the time you would consider me again for a position? That may help you get some feedback. If its not in error, I am sure HR will get back to you to say, the hiring manager wants to talk to you again. In that case, lets say you have a very good chance to making it through this time. If not, they will ask you not to come for another round, which would mean saving time for both you and the organization.