Good question - I just got a CP2000 form from the IRS where they're proposing an assessment of another $12K or so in taxes and penalties on the $25K in "income" PayPal reported for the 2017 tax year (which I already reported through the 1099-MISC section).

I assumed the IRS would be wise to the double-reporting issue by now, but maybe the fact that there's about a $300 discrepancy between my 1099-MISC income and the 1099-K income (various reimbursements from friends) triggered something. I'll update if/when I hear back from the IRS -- I'm asking them how to avoid this in the future. 🙂