If the stock that was sold was already in my W-2 why does it show up again as income

You don't have to pay it again. All you have to do is adjust your basis when going through the 1099-B stock sale information. 

When you purchase stock via the employer incentive program your basis is amount you paid to receive the stock (it might be zero) plus compensation element (W-2, box 12, code V) created either by the purchase or sale of the stock.  If the sale is covered (basis reported), the broker reports purchase price but omits the compensation element amount; so the compensation is reported twice – once via 1099-b as it is and again through W-2.  You must add the compensation to the basis of the stock being sold. If the sale doesn’t report the basis, just enter correct basis. If the form 1099-B reports incorrect basis (the amount without the compensation 

In TurboTax, when going through the stock sale, the quickest way to fix this is just to add correct basis manually Using I’ll enter additional info on my own.

1. Federal Taxes (Personal if SE version)
2. Wages & Income
3. Investment Income
4. Stocks, Mutual Funds, Bonds, Other - continue through your stock sale; when you have entered description, date sold, acquired, sale proceeds and cost or other basis (that you have on form 1099-B), select I'll enter additional info on my own; then enter the correct basis (the basis reported on 1099-B plus your amount from W-2, box 12, code V) in the corrected cost basis field.

TT will report the sale on form 8949 as it is reported by the broker but will have adjustment in column (g) , code B and correct amount or gain/loss.. sometimes TT puts code O instead of code B… - that is acceptable reporting as well.