Hey Glitter.
First you should try and write down the PDF (Hint: Look at a multinomial distribution).
consider n independent trials, each of which results in one of the outcomes 1,...,k with respective probabilities p__{1 ,...,}p__{2, sum of p_i = 1. Show that if all the p_i are small, them the probability that no trial outcome occurs more than once is approximately equal to }_{exp(-n(n-1)sum of ((p_i)^2)/2)}