Running simulations on high performing computing cluster generates the same results

I am performing Gillespie simulations a thousand times which take a very long time. I decided to run 10 scripts each with 100 simulations on high performing computing clusters of my school. However, once I did this and checked the results, the list of the results from the first script are the same as the second script. Meaning if I run the same scripts twice I get the exact same list of results back. Why is this happening given the fact that Gillespie simulations generate random results?