I just received a second CP2000 for TY2017. I can't tell if it is a revision to the first letter or a new bill. They provide no information explaining why i getting a second one. I paid the first one. The second letter is a different amount, but in the same ballpark...$5500 range. I can't believe that on an AGI of $35,000 i could owe $10,000 in addition to the $9,000 I paid through my initial 1040 and payment with the filing.
The CP2000 notice should tell you exactly what the income was that the IRS received and was not reported on your tax return or you reported certain income on your tax return incorrectly. This notice also may be incorrect since you responded to the first CP2000 notice. Your best bet now is to call the IRS using the number provided on the notice and try to find out exactly what is going on with your tax filing.
IRS website for a CP2000 notice - https://www.irs.gov/individuals/understanding-your-cp2000-notice
$9,000 would probably be about right if your were single, didn't itemize and all your income was from self employment - $4000 in regular income taxes and $5,000 in self-employment tax.
one way I can think of you owing an additional $5- $10,000, if the IRS thinks you failed to report all your income. This could be just a pure mistake on their part or someone filed a income form with your social security number. before calling the IRS you may want to go to its website and get a transcript for 2018 to see what the IRS changed from what you filed.