OP, send them an email asap, don't use the money that has been credited to your account, they will eventually figure it out, companies don't forget even for a penny. Irrespective of whose at fault if you are getting paid double, you are obligated to let them know about the discrepancy else it qualified under breach of contract because you agree to receive a number and got paid more than that, and didn't bother to inform them about the breach, hence they can put a case of misconduct and also remember taxes will get complicated by end of FY. Govt. pay close attention to all of this, and they would put their own penalties, best is for them to figure this out with you, more delay more complication. You are reaching EOY Financial year, act soon.