Unfortunately it is too good to be true.
---NOI is overstated because you are not accounting for expenses from the basics (paint, cleaning, etc.) to the expensive (roof, hvac, electrical, etc.).
---NOI is overstated because vacancy isn't accounted for. Go with a 10% vacancy number to give you an idea of lost income.
---Taxes and insurance on this duplex seem very low. $~1.2k in taxes AND insurance per year for a $80,000 property? I don't think so. I bet the taxes are also calculated based on owner-occupied (you living in one side).
---You will NOT be collecting $880 month to do nothing. Remember you will be paying principal and interest on the loan, around $450/month. This drops you down to $430 a month IF you have NO expenses EVER and you NEVER have a vacancy!! Remember the taxes/insurance are probably understated.
Based on the numbers I ran in my head accounting for vacancy, repairs, reasonable PITI, etc. it looks like you wouldn't be cashflowing too much on this property that is almost 40 years old.
The seller would have to come down a very large amount for this to be a good deal. I'd pass on this one.