If filing Form 1, I'm assuming you are a resident of Massachusetts and filing a nonresident return for Connecticut. Your income would be taxable in Connecticut if earned there. It would also be taxable in Massachusetts and show up on Line 3.
However, if you go though the MA return, you should get to a screen, Taxes Paid to Another State. This will show your taxes paid to CT and the credit you are getting on your MA return. Due to the credit of taxes paid to CT, you are technically not being double taxed on your income. This credit will show on Line 30, Form 1, as well as Schedule OJC.