You did not "already pay tax". What you paid was only withholding, an estimate of the tax due. No piece of income is taxed individually. All your income, including bonuses, is added together at tax filing time. The tax is calculated on that total (after adjustments, deductions, exemptions and credits). Then withholding is subtracted to determine if you owe more or get a refund.
The bonuses will most likely be included on your W-2, particularly if FICA was also withheld
