strange!!!!!!!!!!!!!!!!!!!!!!!!!!!!

Hello,
I got lay-off last month and the company paid me one month servicing two weeks ago. Surprisingly, they deposited me another half month salary to my bank account without notice. By the way, my pay was direct deposit by third party. I guess that this is a mistake.Should I inform the employer and return the money? Is there any legal issue here? Thank you very much for your kind reply!