First, you must contact the employer that provided you with a Second W2 to confirm that one of them is not a corrected form. To make a long story short, each W2 should have a different EIN for each employer. If that is not the case, only one of the W2s can be valid.
It can be quite unnerving to watch your Tax Return drop while you are entering information such as your income. The FAQ below provides a thorough explanation and illustrates how entering additional income affects your return:
https://ttlc.intuit.com/replies/3798403
