You owe 2 taxes on it. You owe regular income tax on all your income (including the Net Profit on Schedule C for your 1099 contractor income). Then you also owe Self Employment tax.
Self Employment tax (Scheduled SE) is automatically generated if a person has $400 or more of net profit from self-employment. You pay 15.3% SE tax on 92.35% of your Net Profit (If it is greater than $400). The 15.3% self employed SE Tax is to pay both the employer part and employee part of Social Security and Medicare (FICA). So you get social security credit for it when you retire. It is in addition to any regular income tax you owe on it.
The SE tax is already included in your tax due or reduced your refund. It is on the 1040 Schedule 2 line 4 which goes to 1040 line 23. The SE tax is in addition to your regular income tax on the net profit.
Where are you seeing the 20%? Is that the total of both taxes?
