My return was accepted & payment deducted from my bank account. But I just received notice saying I owe the same amount due to "misapplied" payment. What does this mean?

The IRS took your payment and applied the money to something else on your tax account.  When this happens the IRS instructs taxpayers to call the IRS and request an account review.

You may have a difficult time getting through to the IRS at this time.  If you are able to make another payment, I would do so.  You can always work out the issue next week or via the mail as you will receive communication from the IRS about this matter.  Click the IRS link below for more information.

https://www.irs.gov/individuals/understanding-your-cp260-notice