From the Australian fair work ombudsmen:
When overpayments occur, the employer and employee should discuss and agree on a repayment arrangement.
If the employee agrees to repay the money, a written agreement should be made which sets out the:
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reason for the overpayment
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amount of money overpaid
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way repayments will be made (for example, cash, cheque or electronic transfer) and how often (this has to be reasonable).
Granted, this is generally for overpayment of wages and not specific to redundancy pay. There may be clauses in their contracts with varying terms that allow them to reclaim, but I’m just some dude on the internet.
I also don’t have any studies, but I am aware of xylitol being used in toothpaste, chewing gum, etc., usually with the “assists in the prevention of tooth decay” type of tagline.