Good point, Jason.
Currently, it is random, and repeating tweets may occur.
Suppose there was mechanism in place that would ensure a unique series of tweets until exhaustion of the products, they would still repeat after finishing a round. Statistically, the larger the stock, the less likely to have repeating tweets. If you have a small stock, you would have a high number of repeating tweets in both scenarios.
I am running autotweet for Etsy users for six months now, and nobody has raised this as an issue. I would implement such a feature though if a customer required it.
Thank you for the excellent suggestion.