Dear Kalpana,
Punching/swiping another employee's card or signing on behalf of another employee is far more serious offence and don't stop at just giving warning letter. If you give just the warning letter, it will send a wrong message to one and all that management takes lenient view of the misconduct of this type.
I recommend conducting domestic enquiry and if the employee is found guilty then he may be put on suspension for 1-2 days or forfeit his wages for 1-2 days.
You may re-consider your decision.
Ok...
Dinesh V Divekar
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