Why does TurboTax keep adding form 8959 to our filing when we don't exceed the threshold?

You are seeing this because the amount of Medicare taxes withheld in box 6 is Not exactly 1.45% of the Medicare wages in box 5.  Remove any cents for the Medicare wages and see if the upgrade message goes away.

OR -

Delete the w-2 and re-enter.  Make sure the amount in box 6 is 1.45% or less of the amount in box 5.

I tried all of that. Did not work at all.

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@Mariapaz84 wrote:

I tried all of that. Did not work at all.

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What is the amount in box 5 of your W-2?  What is the amount in box 6 of your W-2?