`/dev/urandom` isn't a real file / stream. It's part of the 'everything is a a file' *nix mantra. Even if two users are reading from /dev/urandom simultaneously, they'll each get unique values. The CSPRNG keeps track of a sequence number and so you'll end up with something like [process 0 requests sequence 0, process 1 requests sequence 1, process 1 requests sequence 2, proceess 0 requests sequence 3...].