Discounting After-tax CF with After-tax discount rate

Hey guys,

I am confused.
I found some training questions in which I have to discount cash flows.
The solutions discount the calculated after-tax cash flows by a tax-adjusted discount rate, but aren't we double-counting the tax impact then?

Example:
After-tax CF y0: -3000 (Investment)
After-tax CF y1: 600
After-tax CF y2: 1556
After-tax CF y3: 1132,4

Discount rate: 10%, tax rate: 60%
Therefore, adjusted discount rate: 1+(0,1(1-0,6)) = 1,04
NPV (tax adjusted): 22,23..

But I would have discounted with 10% as we already incorporated tax effects in the after-tax CFs. Where am I wrong?

Thank you guys