Work depends only on the seed of the random number generator which is based on the time you start the program and the process ID (basically the two most random things on your machine). There are 4 billion possible random seeds and less than this number of users (so far...) so the chance is extremely small that work will be reproduced. Even though we plan to make 1 billion structures, assume that on average someone makes 1000 structures at a time, so that's only a million random seeds, far less than the 4 billion available