1) how can I confirm I'm not duplicating my 1099-MISC income;
For a 1099-MISC, regardless of where you enter it, it will "appear" in at least 2 sections, sometimes three. Nothing unusual there at all.
2) why is the MISC handled differently than the NEC (since both are self-employed income sources)?
The 1099-MISC for earned income appears to be in the process of being phased out, to be permanently replaced with the 1099-NEC. So while the 1099-MISC for 2020 *can* show self-employed or other earned income, the 1099-NEC is *only* for earned income.
