Don't spend it.
If you have no claim against them, and they have no obligation to pay you, there is no possible right you have to keep the money.
While some people might deposit the checks and see if the company or its payroll service catches the mistake, 95% of the time the mistakes are caught. And 10% of the time the company reports it to the police and 100% who catch the mistake at least note it in your personnel records and most of them tell anyone who asks for a reference.
As so many firms these days use outside payroll services such as ADP, the pauroll services know exactly what to do and this can really get nasty.
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