If the money were paid by mistake - they should be paid back.
How they would be paid - should be decided between an employee and an employer.
If there is a problem for the employee to pay back in full - they should negotiate the reasonable payment plan - such as deducting from each paycheck, or any other way they both agree.
But the botXXXXX XXXXXne - if the money do not belong to the employee - they should be paid back.
Let me know if you need any help.