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Now it works. But WHY?

Because that's the way random number generators works. They're not random at all, they generate completely predictable number sequences from a "seed": each time you use the same seed, you get the exact same sequence. In your case, you used os.time() as a seed, which is an integral number of seconds elapsed since some fixed date, and the fact you saw the same number ten times simply means your code was ran ten times within the same second.

I know this and I have triggerd this by hand. So there is a lot of time between. I have purged files between every run and I got the same result if I wait 1 second, 1 minute or 1 day. Wolfgang