Received 2 W-2s from the same company - 1 is from 2016, the other is from 2015. Had already received a W-2 from this company in 2015. This one is different. What to do?

I don't understand why I'm receiving a 2015 W-2 for 2016, and why it doesn't match the W-2 I'd already received from this company in 2015. Please advise on how to treat this W-2.