You need to call your former employer and ask if the 2017 W-2 included this "last check". If not, then you didn't pay tax on this income last year so you should add the W-2 and pay tax on that last check this year.
If, however, your employer reported that last check on both your 2017 W-2 and this new 2018 W-2, then you need to ask them to issue a "Corrected" W-2 for 2018. This will mean that they will reduce everything to zero.
The point of this exercise is to get your employer to tell the IRS that the second W-2 was a mistake. Otherwise, all the IRS knows is that the company paid you this money in two separate years, and you owe tax on it.
As for which year...Technically, your employer should have added it to 2017, but they probably didn't even if they paid it to you in 2017. If this last check wasn't on your 2017 W-2, then you're not out anything, but in fact, got a year's grace to pay tax on it. If this is the case, just add it to your 2018 tax return and be done with it.
