Many Americans have gotten CP14 notices– despite having already paid.

Luckily, this is an IRS error and it won’t cost you.
Stimulus: 200,000 rebate checks worth $850 being sent automatically this week
What is a CP14 notice?
A CP14 notice is to inform taxpayers that they haven’t paid enough, or any of their tax bill. The IRS is incredibly backlogged, which means many people who have paid, haven’t had their official proof of payment registered yet.
If your records show that the correct amount was paid, but still got a CP14 notice, you have 60 days to contact the IRS to tell them you are contesting the notice.
In the chance that your notice wasn’t a mistake– you can either pay in full or sign up for a payment plan with the IRS.