A 32-bit PRNG will inevitably repeat after at most 2**32 numbers. It will usually repeat some numbers much earlier than that. And if you are using fewer bits (eg. rand( 100 ) it will repeat even earlier still. This is both inevitable and desirable. If it didn't repeat occasionally, it wouldn't be random!

The significant statistic is the number of rands you can obtain before the sequence starts to repeat. This is called the period. For the MT, the period is 2¹⁹⁹³⁷ - 1. Which means you could draw a new rand every nanosecond for the next 10⁶⁰⁰⁰ years, and still not see the sequence repeat.