TIL about the 2018 Samsung fat-finger error, in which an employee of Samsung Securities mistakenly distributed shares worth US$100 billion to employees.
I don't know Korea's laws but this would likely be illegal in the US, too. It depends on whether it could be proven that you knew that the stocks were issued in error. And even if it wasn't a criminal act, the company would be within their rights to recover the mistakenly issued stocks.