Catalytic Converter issue

Hi, I wonder if you can help me. I have got a Hyundai Coupe 2l Evo Xreg (2000) which has done about 86000 miles. A couple of years ago I had to have a new catalytic converter fitted in order to sort out an emmissions problem, which cost a lot of money with a local mechanic. It failed its MOT last year due to the emmissions so the mechanic tweaked it and it finally passed. On Friday last week my car failed its MOT again on an emmissions problem and the mechanic is now telling me that it could be the cat again and I might need another new one. I don't do very many miles, and they tend to only be short journeys. My query is whether a cat should have lasted longer than two years or if the mechanic could have fitted it incorrectly or whether the fact that I do short journey could have affected it. The mechanic is taking the car tomorrow to have a closer look so if I could be armed with some information that would be great! Thanks so much for any help you can offer. Kind Regards, Karen